First,[u] [b]no, incest is not common or acceptable in the US[/b].[/u]
Second, you are involved in, and (dare I say) considering marriage into, a very, very unhealthy situation.
I would strong advise against continuing any type of relationship with this guy… And to immediately delete from your psyche any thoughts about marrying him. What I would recommend you do is encourage him to get some professional help.