
If I understood the dynamic correctly, then I feel bad for Derrick. To an extent.
Now, before you lynch me, shit is still weird. But when you think about it, it makes sense. Derrick was older when Penelope was brought in, old enough to be less accepting of her, even without adding his guilt over losing Yvonne. Put those two together and it's no wonder he never saw her as his sister. To him, she was an unwelcomed intruder in his home. He more than likely didn't even spend a lot of time with her/didn't see her regularly. Time passes, suddenly she's different, he's intrigued by the change in her (or suspicious of her) and pays closer attention, enough to see her for the attractive woman she is and then, oh no, he finds her attractive. He knows it's wrong so he represses it, he makes wrong choices and keeps making wrong choices and now we're here.
He has no one to blame for his actions, save for himself. While you can't help who you like, you can definitely help how you react to it. And you can certainly get. Over. It.