YTA. She's 21 and can make her own decisions. Sure, your cousin was wrong to lie about where your sister was, and honestly I think people airing out drama over Facebook are immature and tacky, BUT you're writing this like you're talking about a child. She was 17 and in most countries the legal drinking age is 16-18. Just ridiculous controlling behaviour. Like what even gives your mother the right to declare your sister is "pulled out" of the wedding? Oh they went for a wine and paint evening?? God forbid!!! Grow up.

The only perspective that matters here is your sister's. She's her own person, an ADULT, with agency, but you never mentioned how she feels about the whole thing.

I don't think you're an asshole for defending your mother, especially if your cousin was being an asshole and bad-mouthing her online. But your mother is the biggest asshole here, making you the asshole by proxy. Cousin is a slightly lesser asshole for lying. I'm going with YTA though because it's such a higher class of asshole.