I'm going to get shit for this probably but I think like 40%. The biggest reasons why I saw this is because Chris has a loose grasp at reality and the permanence of his actions, that and along with he lacks an inate ability to decide right from wrong unless he is forced to face the actions afterwards.
On the first part, I think most people can say that Chris is beyond not right, even I'd think a good majority would look at how he interacts with reality and his fantasy and agree that he genuinely believes that one is completely interchangeable with the other. That said, I don't think it is a huge reach to say that his comprehension with an act such as incest and even rape is not aligned with what the average person would have, even people with excessive problems such as mild autism or even mild down syndrome. His value of this isn’t in line with the norm, and with how poorly adapting to reality and that other inappropriate behavior came out on Barb's actions, it doesn’t stretch far to identify it with immoral or inappropriate. The other part is that I do not personally think he sees Barb as the woman at the end of her life and barely gripping at reality, I think his image formed in his mind from 40 years of life with her is absolute and unchanging with time, at least based on his interactions with her in the last 6 or so years.
Second part is I don't personally think Chris has a grasp on most immoral things. This is just a think with people with autism from what I understand, but empathy and morality are both hard concepts to grasp, and where some more direct things like murder is off the table, as we've seen before, certain things like assault and thievery are much less defined as immoral unless there is a punishment to it. Combined with an ego that thinks he's infallible, it creates an interesting thought of whether he legitimately thinks that what he did was rape at all, let alone the incest which from other events I'm pretty sure he doesn't think it is immoral.
I think between these things he really doesn't hold a full grasp on the gravity of what he's done, however there were things that we contrary to this like lying to Null about who it was (shows he has at least a grasp at what society as a whole thinks about incest) and him admitting is was told to stop. There are things there that absolutely say that on a level he knew things were not normal, but to the extent of being wrong, I'm not sure. I also don't think jlhe took her saying it hurts as a strict no, rather a need for adjustment, but regardless, to ignore that, no, she didn't want this, is despicable. Incest aside, even the rape aside, when he stole money from his mom's account and lied to Null, he showed that he was aware he shouldn't have done that, which is an argument for accountability.
Tl;dr he is sick enough that I don't think he had a full grasp on his actions and the morality to it, but there is enough to say that l yeah, he has a good amount of responsibility and should have known better than fucking his mom