The one thing that kinda makes you think at least Bob can't be his dad is because he's so small. If the demi-demon thing runs through the male family line, every son would have to be taller than the dad. So, maybe it's like a recessive gene that only activates every couple generations (If that's how it works, no clue about biology.)
Yeah, I had considered Bob might be a step-dad or something. Honestly, I stopped playing the game after the second forced sex scene where the girl does nothing but talk about taking other guys' dicks. Not really my thing. Don't care for forced sex scenes or hearing about girls' past sex lives.
I get that succubi are unlikely to be virgins(though I prefer when they are), but I really don't like to hear them *constantly* talking about other guys' dicks during sex scenes; it's a huge turn-off. Ended up just quitting the game during the second such forced scene. I was actually quite disappointed, since the game's backstory made for a perfect excuse to have virgin succubi, since the succubi are now born with enough life energy that they don't have to have sex until they're "of legal age" as they put it. Very disappointing that, despite all that, they've still had more trains run on them than the Northeast Corridor by the time they meet MC.
I just consider the Mary ending canon, and consider it a fun, short game that is now completed.