If you want to send your aunt and uncle a gift, then send it to them with a note that you are sorry you cannot attend the party.
Or just send them a nice anniversary card and tell them you are sorry to miss the celebration.
I don't think you should give anyone a dime! How dare they say you owe money and your kids own money. No way!
And I think I would block this cousin from email, facebook, texting - whatever you can block her from - you shouldn't have to deal with them. Or if you can't block them, just delete - don't even open.