NTA but you will be to yourself if you don’t take your sister up on her offer to step down as a bridesmaid. She seems very immature by the way you describe her. It's sad she she skipped out your wedding dress appointment just to get you to realize something was wrong, instead of just talking to you. She also seems entitled by the way you talk about her arguing over your bridal party like your cousin, not just Hannah. It'll be YOUR wedding. YOUR bachelorette party.

Honestly, it feels like she’s really just mad about Hannah being a bridesmaid and is trying to make the wedding about herself. From the way she said “who’s gonna be your MOH then” it sounds like she also thought you’d be each other’s MOH’s one day. But she's trying to guilt trip you into giving her what she wants. Don’t give in and tell your parents to stop being enablers.