I'd have to say go with skipping the party, and seeing the family , it sounds like both you and her would prefer to have some family time, the guestlist for the party, combined with easy alcohol is a recipe for disaster, and it's a party, they happen, and they'll happen again.
the friend needs to understand that family members are coming that she'll only get to see very rarely and she really can't miss it.
Pleasantly surprised how many people are choosing the family first, tho I think like me its more the impending doom of that party, more than any choice of friends over family, helping us make that decision.
Normally I'd pick friends over family, as friends are the ones you chose, but really, this time, I think being there with family is the better deal.