A woman is exposed to radiation and winds up giving birth to two daughters both with the same personality but different sexual fantasies likes and dislikes. They also grew to the age of 17 and stopped aging. They hate her and want her boyfriend.
She is not going to tell her current boyfriend about them and when they meet him they seduce him while they think it is her(they are her clones) They can or cannot have the same personality but if playing multiple char bothers you make it the same personality except while they are in bed.
The different sexual personalities confuses him to say the least but loves her dearly. One may be aggressive the other very submissive. You decide but make it that the mother and two daughters would never do the same things in bed and would be insulted if asked to do so with the exception of the mother who would never be insulted as she loves him too much.
How it goes from hear I'm not sure but I would love some ideas.