I think based on the comments you’ve left that you’re very protective of your niece and as an aunt I understand that. Your niece is an orphan at 15. I find it really crazy that people are shocked that a 15yo with that much trauma is not a rainbow of sunshine to her peers.

I’m going to go against the grain and go with NTA here, as long as you are aware that your niece needs more than a weekend getaway. She needs actual therapy if her trauma is making her lash out and hurt others. Is there a way you can talk to her guardian about that?

As far as step niece goes, the fact that she still wants to go on the trip with the niece leads me to believe the bullying is not that bad and maybe SIL is overreacting. When I was in high school you could not pay me to go anywhere with my bullies.

At the end of the day, it’s your thing with your niece. And frankly it sounds like your niece needs you more than your step niece. Maybe I’m an asshole too but if. I was basically given an ultimatum I would also pick my niece that has been severely traumatized and needs support over the other one.