"Last summer she tried to talk my daughter into eating rabbit pellets telling her they were berries."

OKAY, I CONFESS! I bribed my brother with $1 to eat a dog biscuit when we were kids. Raisins also look suspiciously like dead flies - so I put a few raisins in the window sill, right next to the dead flies, and carefully picked the raisins out and ate them while I convinced him to eat the dead flies... Sometimes I would volunteer to play hide and seek with him and then "forget" to find him...

It's amazing that he survived having an older sister. It's more amazing that we're the best of friends although every few months he reminds me that I still owe him that $1 (I've probably paid him about $30 over the years!!)!

Kids will be kids and some kids have quite a bit of imagination!!

I think your sd's behavior is a combo of things. First of all 9 year olds are emotional anyways, so the rudeness and "world revolves around me attitude" is part of the territory. The key to surviving it is letting her know that behavior is not allowed. If she talks to you or dh rudely then ignore her or ask her to restate what she just said in a nice way. You may even have to help her rephrase what she said.

And with ADHD does come impulsive behavior. At times an ADHD child really can not control what they say or do, it just comes out! You just have to reward and praise her good behavior. With the bad behavior pick specific behaviors to work on. ADHD children can become overwhelmed easily if you pick on each thing they do "wrong".

Good luck and try to be patient (I know it can be hard!!) I have a sd that lives with us full-time and has ADHD and does not take medication.

I agree, some of it sounds like normal 9 year old but if you want to change some of her behavior in your house, you have to have set rules & be consistent. Ignore bad behavior & reward good behavior. By 'ignore bad behavior', I mean if she breaks a rule, go to the set consequence... administer it & move on. Don't spend time lecturing her, asking her if she does this at school or mom's house, etc. That's giving her attention... albeit negative attention.

It would also be advisable to memorize the phrase "but in our house, we don't do that." You have no control what mom does in her house or how SD treats her SF, but you have to take control of your house. She's using many tactics to manipulate... saying she's being singled out is one. Her consequences are for her actions. It has nothing to do with anyone else... you broke this rule, you have time out. Period. By her redirecting the attention to 'the other kids don't get in trouble', she's trying to change the focus away from what she did wrong.

I agree with the posters above. Make a list of expectations. Make a list of unacceptable actions and set consequences for each. Then determine a 'reward' if expectations are met and consequences aren't handed out - a movie night, maybe a manicure girls day, etc. Then stick to it faithfully. Repeat what you do at your house, and make it consistent. Ignore her plays for attention or deflection. Praise the times that go well - even the little things.

And know that 9 year old girls can be selfish, self centered and mean. They are entering a tough couple years - it's a good time to start steering them in a positive direction.

Perhaps she needs to spend more one on one time with her father. It is not surprising that she is jealous of her siblings, who have both of their parents, including her father, full-time.

The ADHD people I've met have always been articulate, friendly, & charming...
unless they're wound up 'way too tight, in which case they're fretful, whiny, & wild, but not deliberately rude & insulting.

It sounds like she's been coached or programmed.

Hubs has to deal with this one, absolutely & resolutely & firmly insisting that she behave decently to you & to her siblings.

He has to enforce zero tolerance;
if he says one more outburst & you get no tv tonight, one more outburst has to result in no tv tonight.

If she throws a tantrum or argues with him over no tv tonight, that's a new infraction for him to *deal with*, not to give in over.

I do feel for her;
it sounds like she's being manipulated, & in the future she'll be bewildered & ashamed when she thinks back on her behavior.

I wish you the best.

FDH has a 9 yo son (I call him A__) with ADHD. We have him roughly a third of the time (usually 2-3 days a week) and have made HUGE progress in the last year with his behavior.
I'm going to go through your post bit by bit, so this will get long and may not be what you want to hear.

She has been tested and has ADHD and is on medication for it.
Ritalin is NOT an ADHD panacea. It manages some of the symptoms, but doesn't teach kids to make good decisions or manage their impulses. You can't rely on meds to change their behaviors - but the meds can help the people involved.
We tell A__ that his meds are to help him learn to manage his ADHD without meds. But the work of teaching him to manage it is up to us.

The one thing that we are really having a problem with is her mouth. She is completely rude!
Most 9yos have a bit of lip. It's pretty normal for a kid that age to have some rude moments.
When A__ gets mouthy, we say something like "Try that again without the attitude" or "Please say that again in a more polite way"

We both talk to her about it but her response is "people need to change for me, I do not need to change for them".
It sounds like one of two things is happening:
1) Someone (maybe BM? maybe her doctor? maybe her teacher?) has given her the idea that she is "broken" and that is an excuse to not try to act like a proper person.
or 2) The talking isn't getting through the "ADHD noise" and into her brain
I don't know how you would deal with #1.
But for #2 you need to revamp how you talk to her. Lectures do not work on ADHD kids. With A__, if we talk at length to him, he glazes over and then interrupts with inane 9yo blather ("Andguesswhat on the computer blah blah blah"). If we keep our talking to Her and I really but heads a lot and I do not know how to fix this.
I love A__ to itty bitty bits, he loves me too, and we have a great relationship. BUT this is work for us both. We have an agreement that we both have to put the effort in and then we both get to enjoy a good relationship. My "jobs" include providing care for A__, and my "perks" include spending lots of fun time with him. His "jobs" include listening to me, and his "perks" include spending lots of fun time with me.
We have to revisit this deal every few months, and it took some building to get there, but I would recommend you try to make an agreement like that with her.

As she has gotten older she has gotten worse with the attitude and mouth.
That's pretty normal.

Her mother recently got married. I have asked my SD if she treats her Step dad as bad as she treats me and she will tell you no.
We're on the other end of that - we're about to get married, and A__ and I get along well. But BM has a BF who A__ has problems with.
A__ and I get long because we CHOOSE to, and FDH supports us having our own relationship. BF and A__ have not made that choice, and BM doesn't allow them the freedom to develop their own dynamic.
I don't know what the differences between BM's and your house, but what can you think of that is different? Could any of those things be causing the disparity between how she acts between you and SF?

She thinks the world should revolve around her.
Of COURSE she does! She's 9! 9yos are egocentric.

She complains that she is the only one that gets put in time out and the only one that gets yelled at. I am harder on my own children then I am on her.
We hear that from A__ too and we don't have other kids!
One thing we found with A__ and time-outs: he needs to be the one making the choice to behave or the time-out has no effect. So if he goes to his room "for ten minutes" he comes out in the same bad mood as when he went in. If he goes "until he's ready to apologize/be polite/be nice/etc" then there is a change in him when he comes out.
Once he sat in his room for an hour and a half. Sometimes he comes out two minutes later with a smile and a new attitude. Sometimes he sobs his eyes out or yells for ten minutes and then he calms down and eventually appears, much improved.
I'd really recommend trying that on SD when she goes for time outs. It makes a big difference.

I dont know how to deal with her attitude and mouth.
I don't know of anyone who DOES know what to do about their kids all the time. We're fumbling through things with A__, without all the answers, trying things that don't work until we find what does.

Plus to be very honest, I do not trust her. She will convince my children into doing things they are not supposed to and then they get into trouble.
Most kids try to get their siblings in trouble.
But please try to learn to trust her. This will be work for you both, but A__ has learned that, with us, if he is honest or "tells on himself" we might be annoyed or frustrated, but he won't be in trouble. So it's OK not to eat your lunch at school (the meds kill his appetite) but it's not OK to throw your lunch out and say you ate it.

Last summer she tried to talk my daughter into eating rabbit pellets telling her they were berries. I have told my husband that if something doesnt change soon, we will be bailing her out of jail when she is 15.
A friend's older sister made her eat a goldfish and is now a successful 32yo businessperson. She hasn't spent any time in prison at all.

The big problem is, she does not live with us and if you ask anyone about her they say that her problems are cuz of her ADHD. I do not believe this; I think it is lack of discipline at home. How can you fix the problem when you only have her part time?
ADHD is just an obstacle that makes problems bigger and more difficult to overcome. It is not a reason to not try. Nor is it the cause of all the problems.

Short answer: Slowly and with a LOT of work.

Long answer that has worked for us: We started off by making a set of "rules for our family" that has rules about being nice and telling the truth. We went through the rules with examples of following and not following, and then started invoking them as needed. That way, he gradually learned the standard of behavior we expected and we constructively helped him find how to meet that standard. The first few hours of A__ coming over were never good, but after he adjusted to having to live up to expectations, he'd get right into the good habits.

We coupled the new rules and expectations with ADHD things like a timer for the computer or until bedtime and so on. We had to use the timer for practically everything for about 4 months, until he got a bit of a concept of time and learned a few time-management techniques. Learning time-management was harder for A__ than for other kids, because of his ADHD, but he did it. We also used lots of other little ADHD management things (checklists, routines, etc)

Learning good behaviors is harder for kids with ADHD because they can't give a "neurological self-reward" just for making a good choice. They need to get an external stimulus that gives their brain that reward until they internalize the behavior into a habit or into their nature. So we had to give positive feedback for EVERYTHING (I got so sick of high-fives) for MONTHS. "You hung up the tea-towel when you were done drying dishes! Yay!! Gimme a high-five!"
Nowadays we still give a LOT of positive feedback, but it's different: "Thanks for putting the tea towel away" instead of the whooping and cheering.

Even once A__ learned to meet behavioral expectations, he still did a lot of things to get negative attention. We talked with him about "good or bad attention" and what each is. So A__'s definition is "good attention" is doing something that someone might give you a compliment for... and "bad attention" is doing something that someone might tell you to stop. We brainstorm with him on ways to get good attention. We ask him "Are you looking for good or bad attention?" or "Can you think of a way to get good attention instead?" and it has gradually worked.
I'd really suggest this if she seeks negative attention, as most ADHD kids do. See, their brains take all attention as positive because good or bad, it triggers the same chemicals in their brains. So you have to stop giving feedback to negative and give A LOT to positive, until looking for positive attention becomes a habit or part of their nature.
It's slow going, but trust me that it can be done.

We stopped engaging in drama. If A__ screams "I hate you! You're the worst dad in the world! I wish I was dead!" because he was asked to brush his teeth, we respond with a quiet "It's too bad you feel that way, because I love you and am glad you're alive. Please go brush your teeth now."
If we respond with yelling back, or trying to soothe his anger, he FEEDS on it and the behavior escalates. He does it again the next night when he is told to brush his teeth.
I advise against acknowledging outbursts of drama.

BM has commented to us that A__ is so much better behaved and that she loves how good he is now. But BM does have some structure for him. Her mom, on the other hand, has no rules or expectations and FEEDS bad behavior like you wouldn't believe. So if we pick up A__ from BM's, it's a short changeover to the behavior we expect. If we pick him up from GM's, it's a long adjustment.

Anyhow, it CAN be done with a kid who isn't with you much, and is usually in a less constructive environment. It's just longer and harder.
But believe me, it's worth it.

Very well said Ceph...

I thought again about this because OP mentioned that they only have him every other weekend.... but the point is that when he is at your house, he follows your rules. It must be consistent & he must know what to expect. If you give him a different consequence for the same infraction at different times (or no consequence sometimes), he will never know what to expect. He may think he has a shot of getting away with stuff if it's too inconsistent, then take the chance.

Another thing is when they claim to be confused (about different rules in different places) or forget the rules. This may be true for kids with ADHD, not sure but maybe Ceph or someone familiar with ADHD can answer that, but there are ways to get around that: Posting the rules & taking time to go over them. But, we dealt with that when my SD9 would claim she forgets... she isn't ADHD and she can remember what she WANTS to remember. What I told my SD is: If she takes a karate class one or two days a week, there are still rules she must follow at that place. They may be different than her school and mom's house. Her teacher may have different rules for her classroom than other classes too. Kids learn what is or isn't acceptable at each place. But, consistently holding her accountable for her actions has made the biggest difference.