I don't see what good can come from avoiding and the idea of wishing yourself ill is very disrespectful of yourself, nevermind your fiance's sister!

No one can force you to drink or force you to dance! Her party is not about you. Stand around with a smile plastered on your face, drink water with a slice of lemon in it and you'll get to the other side. Watch the people dancing and hide a grin of happiness that you're not one of them.