I surely hope it's not the father. That'd be quite a cruel move from the author.

What if it's the daughter?? That could be more interesting
I hope it's the son, honestly.
If only so he can see that his family ain't worth trying to save, and he should just run and stop playing the hero, involving those two "investigators" who also seek to simply drain him of money (if I remember chapter 6 correctly).
I get what he's trying to do - but the fact he
hasn't gone to the police, feels like both "dumb anime logic" and just a stupid mistake on his part. And this is
actually going to be more dangerous for him, getting those two involved in this, now that they've straight-up installed cameras all over that house.
He could just walk away after getting
real authorities involved--stay with grandparents, or something, knowing what he knows about his family's dysfunction; but of course that cuts through any potential to ramp up the tension.
I think most likely it's the dad, given the timeline of the chapter with the diner conversation. I think that would obviously suck, but it'd be more ammunition for
him to just dissolve the marriage, take his daughter, and try and save himself any more heartbreak.
Again, wouldn't be the case - he'd try and fight the guy, probably, get his shit kicked,
then get yelled at by his wife, and feel like an even more pathetic excuse for a (hu)man for it.
If we're going the "twist" route, then it'll be the daughter, and
she will definitely create a scene of violence, either physical or emotional - possibly capturing something on video/audio, to play for her father, to then show
him just how important it is to leave, further entrapping him with her.
Really, it's all a shitshow. But I'm personally rooting for the family to break apart, because it's the least painful for everyone involved.
Though I suspect it'll be some manner of family annihilation, potentially with the brother being the only survivor, all said and done.