I think it should be prosecuted for the sake of the those involved, so they could be straightened out and be able to have healthy relationships with someone not of their immediate family. I don't believe they should be sentenced to prison because they won't get the help they need.
The way I see it, if you're engaging in incest, you have trouble establishing relationships with other people, and that makes you very immature and undeveloped in so many ways.
ETA: Now, I know you're thinking if we prosecute those committing incest, then we should prosecute everyone who is (allegedly) mentally ill. Maybe I haven't thought all this out, but just the idea of people shrugging off consensual incest because it "doesn't effect society'' (with the exception of making people's stomachs turn) and "its no one's business" bothers me.
