K.D.
Girlfriend everyone has an idiot in their family. You should just let her and your dad know that the party is for your son and that if she has any respect for your son, then she will act appropriately. Let her know that if she can't say anything nice, then she shouldn't say anything at all. If she is pissed, then so be it, she can stay her tail at home and be pissy. Don't let her come and ruin a good time for your son and the rest of the family, and make sure that she knows that you won't stand for it either. You just have to put your foot down. You should not have to be stressed over how an adult will act at a child's party, sister. Your dad should attend regardless. You would think that she loved your dad and respected him enough not to act a fool at his Grandson's 1st birthday party, and let her know that as well. Good luck, but be aggressive in what you will and will not tolerate, and make no bones about it.