Why? This is a legitimate question. As I see it, the two primary ethical concerns with incest are (a) if you grew up together with an age difference there's the possibility of grooming, and (b) it's immoral to have incestuous children because they could suffer birth defects and stuff. This scenario avoids both of those— you legitimately fell in love while having no idea you were related, and you're not going to have kids. So whats the problem?