I just had a wild-ish (depends I think).. idea. Brother and sister.. age... I'll let you decide. He is older. Maybe by 5 years or so. And since she can remember he has absolutely adored his sister. Somewhere along the line, though.. it became an obvious lust. He shamelessly makes if obvious he wants her. Sneaks into her room at night.. hugs her and whispers very naughty things into her ear.. tells her ways he wants her. Maybe.. secretly.. she really likes it. Maybe.. he is starting to tempt her. Maybe.. he does.
He would never force her. At least not unless it was play and she was totally willing. He loves her too much. But he does caress her skin.. and kiss her when she least a expects. He finds contact whenever possible. He is possibly quite mad. But he is also her type.. just as she is also his exact type. They are, he keeps telling her.. a perfect match.
If only he can wear her down and make her see how perfect they are. He knows there would be nothing better.
Just a thought. Always ready for bending in whatever way. Doesn't have to be siblings.. any other family type of connection should suffice. If this thought tugs at your dark craving in any way, oh please do tell me.