It's more a question of where would you put the line? Are cousins ok? Are second cousins ok? Third? What about step siblings? How far is far enough? Of course, there is a biological answer, a legal answer, a sociological answer,... and even those depend on who you ask when and where. I can't tell you what I think about it because I haven't been in that situation. Objectively, I think everything is ok as long as you're prepared to live with the consequences.