1. Is it legal to marry your stepbrother/sister?
2. If they are raised together, you would expect them to have the normal feelings that siblings that are in close contact have, not girlfriend/boyfriend and more. That would be incest. ILLEGAL
3. If they rarely/never see each other, anything can happen. I imagine that in this situation, since they really don't know each other and no sibling feelings have evolved, romantic feelings may occur. BUT, I am sure with that if there is a negative response from both parents, they would not explore these feelings.
I personlly don't think it is the right thing to do.
Just my .05.