Should I not go to my brother's surprise birthday party because of this?

He got married last fall his wife is sneaky, controlling and manipulative isolating him from all his friends and turning him against family. She didn't invite his best friend and doesn't want my sister to bring her fianc� so she's not going to go. I want to support my sister's decision and so should I not go either? My mom even told her and her parents how wrong it is not to let my sister bring her fianc� who is my brother's childhood friend so she wants to isolate him fearing my brother will give attention to him over her or whatever her twisted insecure fat mind thinks. Should I make a statement and not go to support my sister and her uninvited fianc�?