It's not like she really thinks of Charlie as her brother. That's what she tells herself, whenever she goes to him at night. He left home before she was old enough to know he'd ever lived there.
God knows they both need the comfort right now. They're the last of their family. No one will ever know. And there'll be no awkward pregnancy to explain, she made sure of that years ago.
And when all the justifications fail, when she's forced to recognise that she's sleeping with her brother, she closes her eyes and imagines black hair instead of red.