Right, but I'll again point out that Chris knows incest is wrong and has stated it himself.
The standard - it'll probably vary by state somewhat, but in general - for an insanity defense is that you either do not understand what you are doing, or that what you are doing is wrong. That has nothing to do with whether you personally think the act is okay, or should be okay, but is about your capacity to understand your actions and the consequences of them.
In the case of Chris, we already know he understands that incest is wrong, and that it is illegal. Therefore, he's not going to get away from this by claiming his mental block made him do it. He understood his actions enough to try, and thankfully fail, to keep them hushed up.
Now maybe they'll put him in a secure institution regardless of that because he's nuts, but I don't know how that gets determined so who knows!