Why should you care? That their choice and their lives to do with what they want. If it makes them happy their sexual choices should be of no concern to you or anyone else but them.

Now, if she was not voluntary into it and/or it started at a very early age then it would be a different matter.

The same with the whole polygamy BS going on right now. We have no right to dictate their religious and sexual practices, let alone kidnap 400 of their children and break up dozens of families because their religious practices/beliefs don't fall in line with ours.

Concerned indifference is whats needed in these cases. Incest is nasty and something I would never consider doing (although I'm perfectly fine with polygamy) but we have no right to interfere with their lives. Give them the stink eye if you don't like what they are doing and move on with YOUR life.