An idea I had while writing my On/Off post started to blossom, and so I open this up to any who would be willing to participate, instead of the narrow idea I had in mind to begin with.
As the title implies, this scenario involves a lot of "closeness" between siblings. My first idea involved an older brother, played by myself, and a pair of younger twin sisters who were competitive with each other attempting to win the brother's favor. Naturally, it would get out of hand, eventually leading to things they have to hide from their parents.
This idea is quite adaptable, though, as long as it doesn't grow to a family of fifty siblings all trying to hide the same secrets from the parents. Ages have to fall within the rule limits, but for the concept to really work well the eldest brother couldn't be much older than 21 and still living at home.
Hopefully I can get at least two sisters in on this!
F/F interaction between the sisters, naturally, is alright. If somehow another brother comes into the picture, I can't see any M/M happening (though look at my Ons/Offs for possibilities, there).
Here's hoping to catch some interest!