If she was warned--especially repeatedly--you should follow through with cancelling the party. Never make a threat you're not willing to back up: the child will know that you're a pushover and she can manipulate you with tantrums.
Your sister's son will get over it in two days.
Originally Posted by
amandamarie 
She was actually warned 3 different times she would lose her party if she did not stop being rude/throwing fits, but I really do not think it registered in her brain that it could really happen. I do feel bad for her when she has a fit like that, but if I baby her for then it seems to just happen again quickly after. I sat in the living room crying when he told her he was not taking her. It may bother me more than her. I just want her have a great birthday party, and then after take away things she likes as punishment, but as I said it is his gift to her so I have no control.