The idea has been inspired by the following line that I saw somewhere (I can't remember where). "Merry Christmas, girls, I got you both a pony. Well, actually, no, I only got one of you a pony. The other one gets coal. Now, let's see those report cards..."
Basically I'm thinking a single father encourages intense competition between his two teenage daughters who punishes and rewards them lavishly (in an overly sexualised manner). The main decider for who gets what? "Let's see who loves daddy more..." Rewards come to the one who pleases him, and punishments to the one who doesn't. I could see possibilities for punishments to include s&m play, orgasm denial, or being forced to pleasure the winning sister. Rewards could cover a wide range of things too; anything from letting the girl go out to the mall and do other 'normal' teenage' stuff, to a sort of 'winner's choice' option.
Much would depend in part on the type of relationship they all share. Is the father controlling to the point where he filters their whole world? Do the girls enjoy the games and contests, or is it something they endure in the way that other teenagers endure household chores? Do they see themselves as doing something for their father? Do they see the way their family interacts as normal, if not to society, then at least to them? I'm kind of inclined towards that last one. It requires a little bit of suspension of disbelief, but then again, the entire premise does.
The father would be played by me, and one of the daughters is already taken by mkkdreams. We'll both have to okay any potential third - it wouldn't necessarily be first-come-first-served.