Well, if "credible" is what you're looking for, then maybe don't search for that in incest-laden porn games?
But OK, let's break this down in terms of credibility. The MC "just turned 18," and is clearly sheltered. He sounds like a kid who didn't get out much and has probably never heard or read anything about personality disorders or any other kind of mental illness. He knew that his mom was "clingy" and difficult, and probably caused the dad to run away, but what young kid has the knowledge and wherewithal to diagnose his parents with mental illness? I wasn't able to put a formal name to my parents' behaviors until I was quite a bit older than 18 and didn't really come to understand them until I became a parent myself.
I get that you feel uneasy about the reveal at the end of the current version. I did, too, for the same reasons, but I don't feel like a rapist because Marge is the one who instigated the whole thing. She suggested the whole role play game. How in the world was the MC--let alone us players--supposed to see through that charade?
Having said that, depending on what happens next, I agree that continued relations between these characters could become (more) problematic.