I found this quite interesting too. He could have said "We're not even related to him." but you're right, he said "You're not..." I'm not sure if it means Jack does find out though, it could just be him using that to get at Kate, who is referring to Aaron as her son even around Jack when he knows that it's Claire's child. (If I remember rightly, Kate was Claire's midwife during Aaron's birth.)
If Jack does find out he's related to Claire though, it'll be interesting to see how it happens. Maybe he follows Christian to and finds Claire with him? Or sees Claire following her Dad only to realise it's also his Dad?
