By the "I'd rather break my leg than go" is just a way of explaining how badly she don't want to go out. I've said things like that before. Like going to Florida with my grandmother a few years ago, I told my husband "I would rather chew off my foot than be stuck in a car with her like that again" And bbt saying "I wander if i can come down with a migriane or diarreah" it sounds like contemplating faking illness rather than wishing it upon herself. But to the original post, don't go. You should'nt have to explain yourself. If you don't like your sister-in-law and she don't like you then it should be understandable why you would'nt want to go. If it makes you feel better about bowing out, then yes fake an illness, play hooky. Or spend time with you fiance and tell your sis-in-law you have to make wedding plans and get everything in order. If you are not the maid of honor then your absense from the party won't be bad. And as far as feeling bad for not going, do you think she would go somewhere for you if she did'nt wnt to go or was uncomfortable going?