[Urwen]

Well, according to Silmarillion, it was 'an evil fruit of Kinslaying'.

Furthermore, there was an instance of first cousins marrying one another: that of Phary and Miriel, where he forced her to marry him against her will, and took the throne that should have been hers in the first place, in accordance with Aldarion's law. And we know how that one ended.

I suspect that, had Maeglin's plan succeeded, both he and Idril would have ended up in the exact same way, sooner or later. The doom of the Noldor would have seen to that, just as Phary's own greed had doomed Numenor and Miriel herself.

In a way, Tuor's actions had spared them from such a fate, even if one of them had to become a sacrifice to save them both.