i'd ask her why she feels she needs to be 1st at everything. i'd try to explain that she doesn't have to be 1st at everything and she's is still loved and the family are still proud of her no matter what. and that sometimes it's nice to make way for big sister to go in the bath 1st as it would make her feel good inside. is there some way people are treating her that makes her feel this way? maybe other influences are at work here and it's not just her.
hoping she gets through this soon for the family's sake as well as her own.