It seems you have already made a decision, but I was going to say it really depends on how much you care for your cousin, will she be offended that you are not there, if it were me, no matter how much i disliked the family, I would go for her sake, if it was likely to upset her that I was not there and that is coming from a social phobic who hates and loathes any mention of the word party, that is just my thoughts.