This is a tough one. Think about it this way: she'll be your sister-in-law and involved in your life permanently now. Maybe the gesture of having her in your wedding party will let her know you really care about having a good relationship with her. Give her enough attention before the wedding and on the wedding day to make her feel comfortable and appreciated, but don't go overboard so that it takes away from you. It may smooth things over with her and also let your man know that you're on his side, too!