Wayyyyy back in the day I started an alternate universe fanfic for Maria-sama ga Miteru suggested by Sean Gaffney based around the premise of what would happen if Yumi had been Sei's petite soeur. I liked the idea but I tend to have a short attention span (of a gnat some would say) and I never finished it.
Looking back on it, I love the premise but I dislike the execution I used. Basically using canon storylines and flipping it around so it's Sei and Yumi. Like the whole thing with Tsutako taking the picture and using a secondary storyline involving the bet for Sachiko to find a petite soeur, which involves Shimako this time. Also, as much as I do super love Sei and Yumi I love them more as friends than romantic partners. I prefer Sachiko/Yumi and Sei/Youko.
Anyway! I started rewriting and plotting out the fic again, flipping things up. I'm still using canon plot events but I'm not really focusing on retelling canon exactly how it is but with the soeurs changed around if that makes any sense. I mean, all the school festivals and the card hunt and elections will happen but I'm not going to tell the stories of how they happen in the same way as it happens on the show.
I find the new version I have in mind way more interesting because it sets up Sachiko and Yumi for romance again but they're more on equal footing since they're boutons then Roses together rather than being grande and petite soeurs so Sachiko comes from a different place with Yumi. Also, by setting up Sei/Youko it goes with a whole White Rose and Red Rose family falling in love with each other sort of joke. Plus Sei and Yumi being so close as soeurs that they like to irritate their perspective partners by being flirty with one another. Or rather that's more Sei being flirty and Yumi putting up with it because it's her onee-sama and she secretly enjoys Sachiko's jealousy.
Mostly I just love the idea of Sei and Yumi being soeurs because I think Sei would instill a lot of confidence in Yumi and Yumi would make Sei so happy in a lot of ways. As for Sachiko and Shimako as soeurs, Shimako is like mellowing to the extreme and Sachiko is so fierce about people she cares for there's no way that Shimako would feel like an outsider about her Buddhist background after Sachiko heard about it.