1. It's not silly. Your mother doesn't like you that much. That hurts at any age.
2. IMHO, she is doing this deliberately. She is a grown adult and responsible for how she behaves. She's behaving in a way that is blatantly unfair and rubs in your face (and in your daughter's face!!!) that she thinks you are a lesser person than your sister, unworthy of the same honor. She owns that behavior.
3. Your sister is grown now. It's one thing to coddle a little child, a different thing to coddle a grown adult. Surely she is not going to OD on baby aspirin at this late date. There's no need to excuse your mom's behavior--even if it's a bad habit, it's still HER bad habit.
She may or may not ever treat you nicer, that's up to her. She may say, "it's my money and I'll spend it as I darn well please," which is absolutely her right. But that doesn't mean you have to put up with her behavior. I don't know how confrontational you are, but I think your mom is overdue for a "come to jesus" talk about how her favoring your sister makes you feel, followed by a calm explanation that if she continues to treat your daughter like a second class family member, you can find more fun ways to spend your holidays.
I was always the black sheep of the family, as was DH, and we both found that relatives suddenly magically learn how to be decent after a few years of nothing more than Xmas cards. And if they don't, bear in mind that ski areas and beaches are open on holidays and there's usually at least one friend who is watching DVDs instead of visiting relatives. There really IS something nicer to do than hanging out with someone who doesn't like you.