Lost!

[bewitched]

Originally Posted by SzczerbiakManiac

I think it's been established in the flash-forwards that Jack knows Claire is his half-sister, so I'm pretty sure he knows Aaron is his nephew.

He learned it when they first got back at his father's funeral. Well, he learned he had a half sister on the same flight named Claire. He seemingly hasn't figured it out yet or I would guess he would have used it against Kate when she refused to see him/go back to the island.