Sometimes she thinks he picked her because she loved Fred, too, because she'd grown up with him and played Quidditch with him and been both the victim and the audience to his pranks and danced with him at the Yule Ball and swapped notes during class and jokes during meals, and George couldn't possibly share any meaningful part of himself with someone who didn't know Fred, too. It's sweet and sad and to be expected.
Sometimes she thinks that he's trying to turn into Fred, to be both of them, and when they're moving together in the dark, she's pretty sure that he's convinced himself that the fact that she did this with Fred means that she can share with him some essential Fredness that he can't get any other way. That's just sick.
But it's a kind of sick she understands, and she's pretty sure that no one who grew up during the war could escape some kind of twistedness, trees that manage to grow even though the force of the relentless wind stunts and twists and bends.
Besides, she's sick enough in her own way: if he ever asked (and he never would, and maybe that's one of the reason she loves him), she'd have to admit that, yes, sometimes she closes her eyes tight and pretends he's his brother, and the thought has entered her mind now and again that her children with Fred might have looked just exactly like her children with George, and even if she loves him for being the man he is, she's still half in love with a boy who's only a memory now.
One way or another, George and Angelina are never alone in the dark.