Maybe something like Twin Sisters each love their brother (Older or younger) very much..more than a family love, they discover. One of them finally decides to make a move on him, and succeeds...without her twin knowing, that is until said twin catches them in bed together.
After that, we can work on it..or work on the whole plot together, if ya like.
She could get jealous and become competition for her twin, or she could want to join in..I dunno. *shrugs*
Anyway..Anyone interested? Just post here if you are..