Here's the background. I have been friends with five girls for over 20 years, we grew up together and as you'd expect our friendships have had their ups and downs but we have all supported each other through a lot at varying times. Two of our group of six are already married and each had the rest of the group as their bridesmaids. It's always been said we would all do this. I am very close to three of the girls who are genuinely lovely supportive friends. The fourth is someone I have nothing in common with and she makes zero effort with me. She's pleasant enough but a bit bossy and totally uninterested in anything that isn't to do with her. She will never come out with us or do anything more than a very occasional quick ten minute coffee and often cancels plans last minute. As such I don't know her very well now although we were close many years ago. The fifth is a very difficult person and has been a bit of a bully in the group but has suffered with depression so whenever she has been rude and unpleasant people accept it and allow her special treatment. She is very bitchy and flits between different people in the group having "favourites" then dropping people over some imagined slight and not speaking to them for months. When she's not being difficult she's good fun and has been a good friend. She has made it clear and actually said she doesn't like my partner at all and bad mouths him, although he doesn't like her either but will be peasant to her for my sake.
My issue is we have all made this kind of pact or verbal agreement that we will be bridesmaids for each other. The group is very intertwined. I'm worried if I don't ask them all then the two I don't ask (possibly plus some of the others) will be very bitchy and angry and it could sever my friendships with them forever. If I ask them all to be bridesmaids it would go along with the status quo and could get our friendships back on track and bring us closer. I feel like I need to have all or nothing. There aren't really any other people I would ask.
Partner thinks just the three I'm closest to but i don't know if it's worth the hassle.