My brother (23) and I (27F) acquired a step-sister (11) (6th grader) 3 years ago. Our parents started dating when she was 5 so my brother was at home and I was at college. My brother and I both happen to be getting married in 2019. My step-sister will be a bridesmaid in my wedding meanwhile my brother does not want to invite her to his wedding. We are from FL but my brother is getting married in Maryland because that is where his fiancés family is from. We doubt my step-sister's mom will let her go to the wedding anyway because she is very controlling.

 My brother and his fiancé decided they wanted their wedding to be 21 and up. They say its because they dont want to worry about kids at the wedding and have them crying during the ceremony etc. He also says he can't invite my step-sister because it wouldn't be fair if she was invited when his fiancés younger cousins are not. Now my brother and mom are in a big fight because he is not inviting our step-sister and they are both texting me about it. This is extra weird to me because they never fight and my brother is usually very laid back. My mom is worried that this will upset my step-dad that his daughter is excluded. She says she doesn't want this to spur resentment between my brother and step-dad Although they got together when we were grown-up they naturally want their children to be a family.

 Is my brother the asshole for not inviting our step-sister and as my mom puts it "his fiancés future sister-in-law" to his wedding? 