K.E.
Don't get mad - it's a waste of time. You can't control other people's actions, you can only control your reactions. Your plans were made long before, so as far as etiquette is concerned, you RSVP with regrets and send a gift. It is impossible to plan around everyone, just as she won't change her plans for you.
Enjoy your vacation and don't let her get to you. Some people don't see family as being overly important. It's not worth you getting upset over - that won't change her. I've wasted a lot of time on agonizing over things others do and say and I've learned that it's just not worth it.
Best of luck,
K.