I will try to keep this quick while still including everything relevant--I (25F) am getting married to my fiancee (25F) in about a year.

For some context on my family dynamics, my sister (also 25F) has been problematic for a long time. Over the years, she has, on multiple occasions, instigated fights between fiancee and I by twisting information or just totally making things up. She can also be very manipulative, which makes it pretty much impossible to see her coming when she wants to stir the pot. The most recent time was almost a year ago, when sister picked a fight between fiancee and I in my parents' home. The evening ended with my father chasing her down the street while she cursed out her boyfriend. Since then, neither fiancee nor I have felt good about her. The relationship has been strained for years, and this was completely over the line. I have become desensitized to her bs, but fiancee is still (rightfully so) really struggling with this incident.

To the wedding--originally, fiancee and I each wanted 2 bridesmaids. About 2 months ago, she asked two friends, and I asked my sister and a close friend that I grew up with. We always planned to have bridesmaids walk down the aisle, a friend would fluff her dress, my friend would fluff mine, then they would all sit in the front row. A week ago, my friend backed out of being a bridesmaid. This was a huge deal for many reasons, but the most relevant one to the wedding is that now we have an uneven number, and we do not want things to look weird going down the aisle (nonnegotiable). So the options were 1) fiancee uninvites her close friends from being bridesmaids or 2) they walk down the aisle, our moms walk with our siblings, and moms fluff our dresses. Fiancee hated both of these options, and I impulsively said that one of her friends could fluff my dress, and the other could do hers.

She LOVED this option. She does not want moms to do it because her mom hates the idea, it would have an awkward flow, etc. (and she is right, it would have a weird flow). The problem is, when we discussed it again today, I realized that I really can't have her friend do mine. My relationship with my sister has been rough, but she is still my twin sister. When it was a lifelong friend fluffing my dress instead of her, it would have caused tension, but been manageable. However, having one of her friends that I have only known for about a year do it who I am not that close with would be incredibly damaging to any future relationship with my sister as well as my and fiancee's relationship with my parents.

To me, this just isn't worth it. I'd rather have my sister do it and maintain peace. I know that in a perfect world, having her friend do it shouldn't matter. But it does matter, and I'm just not wanting to tank my relationships with family over fluffing a dress. But fiancee is very angry and said she's not going if sister does it and is very upset that I am saying no to her friend doing it. AITA?