I want to say no, that's sick, don't ever do that. But... as much as I try, I can't think of a good enough reason not to. Some people here seem to think stepsister is OK, and I think that's sick.
It's not a question of blood relation for me, it's a question of crossing the line. Brother/sister? CROSSING THE LINE. Oedipus-esque horrors? CROSSING THE LINE. First cousins? Usually, CROSSING THE LINE. But in that last case, it does depend. First of all, who would know? Second of all, who would care? I fear that it's just stupid horniness on his part and he's only interested because he's desperate. If that's the case, stop him now. Others here seem to be confused and think he wants to marry her or have children with her... I don't see that as the case.
This is all very much moot, however, if she has shown no interest in him. Wouldn't it be sad if he hardened his resolve (and nothing else) and then tried to hit on her? I mean, geez. That would scar him for life. If she hasn't made an absolutely clear and obvious first move, he should forget about it.