Lots of illicit drugs. :) Seriously, all you can do is set the record straight yourself, and if people choose to believe her over you, that's their problem.

I myself am a coward who likes to avoid confrontation at all costs, so occasionally I make up an illness that gets me out of family gatherings. Although I'm not usually invited to birthday celebrations for SS's given by BM, I just try to stay away from the party as much as possible. I greet whomever needs greeted and then wander off into an empty room. You could also excuse yourself from invitations to Skids' birthdays and later on give them a special card or present if it's really that bad. If they like you, they might be disappointed that you didn't show, but it's not like you want a scene either. As for mutual acquaintances, any friend who takes sides in a breakup or divorce isn't really a friend, so I would try not to care about their opinions.

I avoid her like she has the plague. I am cordial but not pals... nod of the head to acknowledge her presence is all that is needed in my situation... anymore than that and she wants to sit with me and complain how bad her life sucks since she got divorced.

Usually if the "event" is not on her day she won't go. Although today shocked me ... she is in school mornings but I guess she skipped to attend the morning holiday concert at the school ... I went to the morning one figuring she "might" go to the afternoon one. If I had known she would have been at the morning one .. I would have gone to the afternoon one. I didn't see her truck outside ... next time I will drive around and "really check" to see if she is there... but thats in March. No big hurry.

I sat with the other bus moms. Group of about 5.

As far as birthdays my SC think theirs are a national holiday. I do not have parties ... we do dinner and a movie or chuck e cheese just the "family". Mom has the parties. Its "their day" they get to pick where we have dinner what movie we see or chuck e cheese .. whatever they want to do we do. We get invited to the parties .. hubby says he couldn't stand the relatives when they were married why would he socialize with them now. The kids invite her to go with us she hasn't shown up yet.

I am all for separate parties. On SD's last birthday (8th), her mom caused a huge scene at DH's work and her threats to take away SD because she didn't think he was paying enough for the "joint" party (which he was paying more than half) started the "custody battle" that lasted five months and cost us ten grand. (plus whatever it cost her). Then after the court said they could sit down and agree on a holiday schedule, BM asks DH, "what about her birthday party?". She was surprised that he wants to now have a separate party/celebration.

As for other events, you can't always avoid school functions or later on, weddings etc. so I agree with cawfe, be cordial and pleasant. Don't chitchat. She probably doesn't care what you think and may even get upset at what you say or twist it around and use against you later. Some bio mom's can be very sincere and appreciative and if she is, you are lucky. If she isn't, it's best to stay out of her way and not give her more reasons to hate or dislike you.

My BF's son's mom is mostly dismissive of me, with a dash of rude. I don't want to be her best bud, but I don't mind her (and that was also my opinion of her before I got together with J__).
But, since J__ and I were friends before we dated, and they have a few mutual friends, I wind up at a few social functions with her. I'm polite, smile and say "hi", and when we're at the same end of the table I make sure to include her in any conversation. She scowls, 'forgets' my name, and when we're at the same end of the table she specifically changes the topic to people I don't know...
It's frustrating, but at least I know that I'm being nice and not making waves. Also, I get to b17ch to my sister afterwards about how BM was crusty to me, and know that she can't complain about my behavior without being less than truthful (aahhhh, sweet satisfaction!). She wouldn't have a leg to stand on if she said "J__'s girlfriend is NICE to me, what a bag!"
The people who's opinions I actually care about know me well enough to know that I wouldn't be with J__ if I had misgivings about his son and know that I'm rarely rude.

As for A__'s birthdays, I don't know... A__'s last birthday was right before we got together. If it was on 'my turf' (for lack of a better term), like at J__'s place or at his dad's, then I would go, no questions asked. I feel comfortable in those places, and wouldn't feel awkward or out-of-place. If it was at BM's place, or her mom's place, I doubt I would go. But, if I was invited by BM, AND J__ said "It's really important to me, I want your support and company," AND A__ really wanted me there, I think I'd go.
I'd be concerned about 'invading her territory' or the potential of an 'ambush' too, so I'd make sure to be on my VERY best behavior and try to keep fairly quiet.

I'm crossing my fingers for separate parties though!

I think you'd get the best results by saying graciously how 'kind' BioMom has always been to you and how wonderful the kids are. That way, if she's said vicious things to others, she's only made herself look phony and two-faced -- which sadly seems to be the case...

If you're really lucky though, BioMom may get word of how nice you've been, and she'll either change her tune about you or fume at how her own actions have made her look.

Ain't the South grand?

Oooh! That's clever, Sweeby!
I like it!

I used to make every effort to talk to bio-mom at events, introduce her to SD's friend's parents, etc. I would later hear from SD that I was "rubbing it in BM's face that I was friends her the kid's parents." Some may argue that this is only from SD's mouth, but it was verified some time later. Each of my attempts to draw her in to SD's life were turned into my trying to belittle her in some way, so I quit. I may say hello now, or I only smile and go on with my business. I do have a bit of a different situation since my DH is the CP and SD considers me her "real mom," but she does see BM EOW.
I think the high road, though often difficult to travel, is always the best one. Sweeby is right - she will only look the fool if she opts to go low while you stay high.