The father is Peter, her ex-boyfriend, to whom she returns. And no, I don't think she had sex with her brother, though that's what he obviously wanted. It was the knowledge that that could never be and that she'd given herself to another that pushed him over the edge.
As to leaving, I think she was in shock, numb, so appalled by what had happened that she couldn't cope at all. New damage heaped on top of old damage, making her totally non-functional in the moment. As to not wanting to see Steven anymore, how could she after the catastrophic outcome of their totally obssesive behavior? She'd already said it, theirs was not a sitting across the breakfast table sort of connection.
"Sure, lots o' times!"

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Anyone!!!!? Please help me here.
JeSkuNk