I've heard people claim they were "insane conservative fundamentalist christians" but these accounts come almost exclusively from posts defending Nick, or Nick's description of them.
And it should be noted Nick describes anyone who isn't into pedophilia or as obsessed with asses and poop as him as a "stupid closed-minded conservative" and also that his version of Christianity is pretty much completely his own invention, as among other things he believes God will force Anna to marry him when they die if she refuses while alive.
I think at the very least, they're incompetent lazy fucks, because their daughter seems to be growing up just as poorly as Nick. Her teeth are now as nasty as his, and she has massive bags under her eyes indicating poor hygiene and diet. If you have one fuck up kid you could say maybe it's just the kid, but two suggests the parents are at least partially to blame.
Beyond that, we know his parents are divorced and he's been passed from one relative to another until he was left to his own devices in that apartment. As far back as childhood he was getting in trouble for smearing his shit and semen on the walls, and had a criminal arrest record for attacking his mother when she'd try to clean his room. He had a pillow he pretended was Anna which he would occasionally fuck, and it was apparently rancid and nasty as he refused to ever wash it -even though she'd never touched it or had any connection to it. Attempting to throw this away and take down his shrine to Anna was what lead to him attacking his mom.
After being kicked out of both parents houses, he stayed with an aunt and his grandmother for some time until they couldn't put up with his antics any longer -which according to him included hospital visits for getting objects stuck in his ass, refusing to let him keep women's clothing, and attempting to get high by cooking his own shit in the oven, which he believed would be a faster method of creating "jankem".
He was in special ed pretty much from middle school on, which he said he enjoyed because they didn't make him do any work, and he claimed to actively try to make himself dumber (which I feel was an excuse for how stupid he is, claiming it was intentional.)
He would "larp" in school during recess, but the reason everyone stopped hanging out with him was because even in elementary school the way he did this was extremely sexualized (which does suggest he may have been molested.)
In middle school and high school he'd stand around pretending he was freezing objects with his mind and doing karate kicks in the air and swinging around sticks, and kids would gather in a circle around him and laugh.
He describes how for every Christmas he would request specifically anal porn, and how in school he could talk about nothing but anal and shit, leading to much time spent in ISS as he would disrupt class to speak loudly about these things.
His teen years were largely spent stalking Anna online, writing obscene rape fantasies about her, and telling her he was going to make her have his children and then rape them as well, and sexually harassing other underage girls, notably Maddie who brought him to /cow/'s attention to begin with.
I think his family are just happy to be rid of him at this point. They kept pushing him further away not wanting to deal with him, and still finding horrors when they'd take him food or something, and now they don't have to worry about him, they won't hear anymore complaints about him, and he's in the perfect place where he can tell all the other inmates about how much he enjoys pedophilia and anal sex.
https://en.wikipedia.org/wiki/Stephen_Donaldson_(activist)
Read the sections on his arrest for a glimpse into what Nick may be experiencing at this very moment.