Couldn't Niniel and Turin work this out?

[Aiwendil]

Originally Posted by Eorl of Rohan

I mean, brother marries sister in lots of cultures (especially in royal families, to keep the bloodline pure),

Really? The only example I can think of where this has happened is ancient Egypt and I believe I've heard that even there, the extent to which this was a normal practice has been somewhat exaggerated. But of course there may well be other examples I'm not familiar with.

Besides, isn't suicide/infanticide sinful as well?

No one committed infanticide in the story.

As for suicide - personally, I've never understood why this is so widely considered sinful or immoral. Tragic, certainly, but how is voluntarily ending one's own life sinful? However, that's just my view; I expect Tolkien would have agreed that it was sinful. One does wonder what he thought the 'correct' response would have been on both Nienor's and Turin's parts.