That is heartbreaking....i was speechless while reading and i do understand why you do not want to attend.
I do blame your mom for not encouraging things 'together' I know the age difference is large but you are family.
Just a few questions
Did you live with them fully.? Cause that does make a difference with vacations...i take some vacations if not most without my dh's kids...but they do not live with me and i do not want troubel from his ex.
If you didn't maybe your mom felt rejected by you......i dont know enough details...as in...do you have a good relationship with your mom and stepfather? Did you guys fight and that is why you were excluded in the early years?

Either way, you cannt do anything about it. You're brothers have been raised to have a distance from you. I'm sure they think nothing of it because you said your mom facilitates the separation.
Bu ti have to ask, why does she or why did she faciliate that? To me that is not a usual thing to do as a mother.
I want my son to love his half brother and sister...what i'm afraid is that bm will sabatoge that relation for my son. I know the kids love him now...but what abotu later on?...sorry i'm ramling...i just do not think its normal for a mother to separate kids unless she saw something or stepfather pushed her in that direction with many small steps.

I am so sorry for you -- but I think you have to accept that for whatever reason the situation is what it is.

I'm sorry too and agree with KKNY that you will have to accept the situation and also decide what is right for you. If it is too much for you to handle, then don't go. You may want to create some distance for your mental health, it is hard to be rejected by family and to keep on trying for nothing. Sometimes the best thing to do is to take a break and come back with YOUR boundaries and you in control of YOU.

I'm so sorry -- and I can see why you wouldn't want to go. It sounds like another first-class opportunity for your family to reinforce your 'second-class' status, and that's got to hurt.

But I'm going to suggest you go anyway. Say nothing demanding, provocative or accusing before the wedding. Be kind, gracious, loving and supportive. Don't make a scene, drink very little, and if you find yourself tearing up, just mumble something about how "it's all so beautiful" or "my baby brother" or "I always cry at weddings". In other words, be sure your behavior is 100% above reproach so they can't turn it around and blame you.

But while you're there, take notes. Not of every little thing, but of a few concrete examples where you felt slighted. Write them down when you get home, and add in the Christmas, vacation and other examples. (I'd aim for about 5 instances -- too many to dismiss, but not so many that you look irrational or hysterical.)

Then a week or so after the wedding, when presumably, your mother will be able to breathe again, ask to stop by her house and talk to her for a couple of minutes. Try to stay calm, but tell her honestly how hurt you feel at the way you've been treated, and that you want her help to remedy the situation. All of the children are now married adults, so you should now ALL receive about equal treatment.

If you can have this conversation reasonably calmly and not stomp out in furious tears -- If you can do that and it still doesn't work, then I'd agree with the 'semi-estrangement' camp.

OP, could you share what happened when you were 18? Did you go away to college? It may be that you had minimal contct with the younger child.

This is one of those situations where I wish we could hear from the other side. There may be reasons this all came about and I think there are some clues in this sentence......

I just only found out about the Best man situation the other day and childishly sent my little bro a text to see if he'd even care if I wasn't able to attend, to which he basically replied "do what makes you comfortable, fiance and he don't care as their too busy making preparations.

I suppose I could be wrong but really have to wonder of your other childish behaviors while growing up and since then. What I suspect is you spent quite a bit of time establishing your place and esteem or something similar. People who behave this way are usually under the misconception that others cannot figure them out. I also want to point out we have no way of knowing how much of the statement is actually quoting him because you don't end the quote, and wonder if you preferred he beg you to attend. This is what I'm wondering about. Sounds like he is used to your pranks like this. I can't help thinking there are some things we cannot know. I know your feelings are hurt and feel bad for you for that. Again, maybe I'm wrong but it's possible you need to think about your own contribution. Many things are said and done over the years that we choose to forget about. They may feel you have some things to apologize for.

Thermo, you bring up some interesting points.

As to the wedding, I think other people have brought up a similiar issue, and the response was partially where there is a big age difference between step or half siblings, they may not view each other as siblings.

I also think we are missing the history.

"But since I was 18, I have been isolated from them. My parents include both the boys in all their vacations and most activities, although I'm rarely if ever invited. They even started excluding me from their 'family' Christmas with the Big Tree upstairs, since I have kids - supposedly, and have made me do Christmas with them in the downstairs entertainment area with my aunt and her kids - after and separate from their Christmas (although both of my SILs have attended theirs since they first dated my brothers)."

I second KKNY and others who suggested there is a major backstory here that is missing. No parents "isolate" their 18 yo daughter (if that's what they did) without a reason. Parents don't have a "separate" Christmas (with a separate "Big Tree upstairs" which I guess is also an indication of a more special family celebration)from their DD just because she has children. (You indicated that your brothers' girlfriends (now wife and fiancee) have always been included in the "Big Tree" celebration, so it's not the fact that you have a DH.) AND, as for the slide show, it is extremely weird that you were not in any photos. I don't mean to be flippant, but, in the absence of some horrid mistake, a monkey putting together a slide show like this would make sure that there was at least one photo of all of the groom's siblings, especially one who lived with him, as you apparently did, for the first nine years of his life.

No, it seems to me that there is much, much more to this than you have disclosed, so much that I certainly can't give you any advice. However, IF on the other hand, you truly have given a full and accurate depiction of events and you have never done anything whatsoever to warrant or cause your "isolation" and have no idea on earth why your mother, SD and 2 DSBs would treat you this way, then if I were you, I would consider these people psychopathic and disassociate myself from them.

this doesn't sound right. sorry, I don't know what to advice. I would attend and I wouldn't say anything, you can't change it. could it be age difference?

DD's brothers (also half-brothers) are 12 and 20 years yonger than DD. X does a good job cultivating their relationship with DD, every time he calls her to college he puts his older son (the youngest is too young to talk LOL) to talk to her etc. when she visits she spends a lot of time with them. She appears close to them however as they grow up I do not know if DD would be involved with them as much as it would be intact family and less age difference.

BTW at my brother's wedding anniversary last week few of his friends said they didn't know he has a sister (and we are intact family only one year apart in age who live few blocks from each other). I understand that as we got older we have separate set of friends and some of them might not know everyone. But all of my friends know that i have a brother even if some haven't met him. It seemed bizzare to me that he didn't even mention me (his only sibling)to some of his friends or never showed a picture. We do not look alike so some of his friends were sincerly shocked to learn that i am his sister. i felt rather stupid. They weren't close friends but still i was hurt.

jmandersonfinch, I think Sweeby had the best advice. I am sorry this is happening to you.

From OP -- "But since I was 18, I have been isolated from them."

We need to understand why this happened.

No parents "isolate" their 18 yo daughter (if that's what they did) without a reason. Have you seen thurman's posts? Does anyone have any doubt that he would have done exactly that to his SD if his wife allowed it? If OPs mom isn't as willing to stand up to her husband as thurman's wife...

That said, I do not think a guy who asks his closer-to-age brother but not much-older sister to be in the wedding party is at all unusual or insulting, and however your parents have treated you, that is not your little brother's fault. He was just a child; 4 when you were 18? He is also 19 now, just getting ready to start out on his adult life and I think you do have an opportunity here to develop your own relationship with him, separate from anything your parents do or have done. I think that refusing to go to his wedding would pretty effectively shut down that possibility. Unless you are ready to give up on any chance of a relationship with your little brother, who is not really the one responsible for the past, I think you should go. I understand it will be uncomfortable, but it is just one day.