Personal squicks, even illegality, come second to what is canonical for that fandom. I'm squicked by slash and find incest mildly nauseating, but if it's canonical, then there's nothing that we, as PPCers, can do about it.

If not, of course, BURN THEIR SORRY BODIES.

*cough* That said, even if is canon and you can't charge for the incest itself, you could - I'm sure - find a number of nit-picky things to get them for instead.

The Department of Bad Slash counts it as a charge, unless it's canonical. Think about it; incest is considered taboo in most cultures. It's also illegal in most places (up to a point; technically having it off with your first cousin is incest and that is legal). And if offspring occur, it's geneticallly unwise (although that's not *usually* a problem we have in Bad Slash ... although mpreg does seem to be on the rise ... *cowers in fear of Rule 34*).

It depends on the canon, the characters involved, and how well-written the story is. What the fandom thinks has nothing to do with it. As you said, incest in Tolkien is pretty much always a charge, particularly if Elves are involved, because that's how Middle-earth works. Incest between Tolkien's characters would also pretty much always be a charge because of their society and personalities--same with the Weasley twins in Harry Potter.

If you've got a canon or characters with which incest might not be such a strong taboo as it is in our culture, and if the story is well-written in all other respects, then you might let it slide. I'd lay money that doesn't happen very often, though.

~Neshomeh