AITA for not putting my niece in my wedding

Me and my fiancè are getting married. We met young. Funny thing is we were both teen parents when we met. I was 17 and he was 18. He had a son who was 18 months and the mother of his child was in rehab in a completely different state. I was pregnant with my son. When we started dating so many people told us it wouldn't. They told me I'm pregnant and should be worried about that, not dating a boy with a kid. They told him he has a child, he shouldn't be taking on a pregnant woman. We have been together 4 years now and are getting married in March. When putting out wedding together we knew our son's were gonna be in it. My son is gonna be the ring bearer and his son is gonna be a flower boy. One reason why is that's how we met. My step son was 18 months and waddled over to me to give me a flower. So it's repeating history in a way. My sister who is one of my bridesmaids is upset at this and said she figured her daughter would be in my wedding. I never said I was putting her daughter in my wedding so I'm not sure where she got that from. She asked was I really gonna dress my step son in a dress and stuff and I said no. My son is wearing a black suit, and my step son is wearing a white suit. She said that's gonna look stupid and people will think it looks stupid. I said "Okay. I don't really care what people think." She said "Well you should because your wedding is gonna look stupid." I told her if I cared what people thought I wouldn't even be marrying my fiancè right now. My sister doesn't want to be in my wedding anymore now and said I was turning my step son into a f word. I lost it on it with that. Told her to never call my step son something like that ever again and don't bother coming. I don't think I did anything wrong. It's my wedding. It's how I want to do things. And I'm disgusted with her language towards my child. AITA just for not letting my niece be a flower girl