Board Thread:General Discussion/@comment-75.85.184.179-20160620213454/@comment-25405775-20160624053249

Would it be incest from a biological point? Yeah, probably. Would it be incest from a legal point? No, otherwise Tywin wouldn't have married Joanna (his first cousin, her last name was already Lannister even before she married him) without anyone blinking an eye.

So yeah, technically if Jon and Sansa wanted to marry they could, but I doubt that's what either of them wants.