Sister drama

My sister and I don't get along. We never have. Due to a complicated family tree and family history, there's always been jealousy, stress, and high emotions all around. It's not any one person's fault, but as I've gotten older I've tried to be peaceful and haven't really felt that she's done the same. Maybe she's waiting for me to make a bigger move. I am the oldest and I'm sure she probably blames me for our poor relationship.

My mom just told me that my sister wants to be a bridesmaid. My sister never makes herself vulnerable, never tells people how she really feels, so the fact that she's hinting to my mom that she wants to be a bridesmaid is kind of a big deal. I had always assumed she wouldn't be interested.

If we were having a big wedding, I think I'd more readily say yes. I would love to have my fiance's siblings involved (2 sisters, 1 brother), and it would be respectful to have my own siblings involved as well, and there'd be more people to cushion any drama. But I was planning on only having a MOH. Partly to keep the cost low, partly less drama, partly my fiance and I value our best friends and don't have a large friend group.

The trick is that my fiance wants a best man and one other groomsman. We were thinking of maybe cutting one, or maybe having it asymmetrical. I feel like that would be a slap in the face to my sister though, especially if his brother is a groomsman. I don't want to include his siblings if neither of mine are included (though my brother probably couldn't care less). I just don't want to make her feel like she's purposely excluded. But if I do include her, I'm worried about added stress, possibly fighting, etc.

Any and all advice welcome. I'd especially love to hear from people with similar family drama about if they regret their decision to include or exclude a sibling.