Personally I think that because your mother does accept you and is there for you, then you should be there for her. Even if this is just a very short appearance. There will be other people there too right? So you could perhaps avoid your mean older sister? We have a similar (similar in that we are no longer really on speaking terms) with my mother's brother and his wife. We still go to all family functions whether they are present or not, and we just avoid them.
For you, that MIGHT be a bit more difficult since it seems that the majority of your family is being horrible to you....so now I am not quite so sure. I agree that you should talk to your mom and your older sister. If your mom wants you there, maybe just try it for a little while (the more you are present with your family, they more they might appreciate your diagnosis?....I don't know...) Or maybe you make up an excuse (I don't usually like this, but it might be better than being blunt and ruining her birthday if she is sensitive about this), say you cannot make it because you have to do something important that day, and you and your husband can offer to take her to a special dinner another evening??
This is complicated! I hope things work out for you!
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Diagnosed with classic Autism
AQ score= 48
PDD assessment score= 170 (severe PDD)
EQ=8 SQ=93 (Extreme Systemizer)
Alexithymia Quiz=164/185 (high)