Tried to do this one before and it flopped.
This is a four-way role-play for myself, two more ladies, and a gentleman.
Basically I and one of the ladies play identical twin sisters. We have recently moved to town, and just started at the local high school (sixth form college, or local equivalent).
My character is gay, the other twin is straight. And no, this isn't about lesbian twincest! The two sisters are quite happy with each others' sexualities and while they may tease each other occasionally, and even share the odd bath still, there is nothing between them except sisterly love.
No, this is about the other two characters. One boy, one girl. The girl has fallen head over heels mutually in love with my character, and the boy has fallen for my twin sister, who of course feels the same way.
But this is where things get complicated. (And by the way, the bulk of the credit for this idea goes to William S. and/or Francis B). See, the twins are rarely seen together. One of them is artistic and good with her hands, the other scientific and good with her head. One is into running, the other swimming, one likes beef the other chicken. (One likes sausages, the other like kippers...? LOL) Apart from their appearances, they are as different as chalk and cheese.
Because of these different academic and social interests, as I said, they are hardly ever seen together, except at home... So when the young man sees his new sweetheart sitting on a bench eating her packed lunch, he of course goes to talk to her, and is most surprised by her rebuff and lack of interest. The girl sees a boy talking to her sweetheart and gets jealous. Then later that same day, the reverse happens...
The whole thing comes to a head eventually at the home-coming ball, to which each twin is asked by the wrong partner. By now of course, the twins have compared notes and know what is going on, and being somewhat mischievous, resolve to teach their respective partners a lesson in how to tell them apart...!
Just to add more detail here:
This is a romantic comedy, story based rather than smutt based, although there will be chances for smutt. It is strictly two pairs, with no sexual interaction between the boy and the lesbian twin or the girl and the straight twin. The idea is that the mistaken identity creates confusion and flirting between the "wrong" couples is at first rejected, then used as a source of entertainment by the twins.