That makes no sense at all. You obviously wanted them to be there, otherwise you guys would not have sprung for their tickets. You are absolutely NOT being a bridezilla. You want your sister to be there. Does she realize how important it is for you that she be there?
Did you already tell them you guys had travel insurance? A little decetful, but maybe tell them you are going to lose a chunk of your money if the cancel and don't go...