A guy in his early twenties or very late teens is living with his Step-Sister and they're having sex. She doesn't show much if any emotion while having sex. He begins to think that he's not very good in bed. And that his Step-Sister is just having sex with him out of pitty. When the oportunity to have sex comes along with someone who is not part of the family he initially turns it down because he's still unsure of himself.
Eventually he's talked into having sex with that other person. And she thinks he's great. And she wants more. Not believing her he talks to his Step-Sister. He finds out that she's not the kind of person to show emotion during sex and that she actually really enjoys his sex and she is actually pregnant by him.
The other girl may be pregnant by him too.
Or there is no pregnancy and the guy just gets to be with one or both girls.