I'm in a wheelchair & my family doesn't consider this when planning holidays, do I have the right to be upset?

I feel that yo have every right to make this a point! Especially if special exceptions were made for your cousin. Your family could have also considered your wheel chair (which Im sure isn't new news) when they were considering rescheduling for your cousin! I would make a huge fuss about this and would feel the same as you if not worse. People who have disabilities can not help this and should not be looked over just because its easier for the family at the time.Good Luck to you!