It's like a rocket. You need all the parts of a rocket to work or you won't get off the ground. If you are really unlucky, it may even explode
With psychiatric disorders, it's similar. Break a few choice genes and the whole system goes to shit. But unlike a rocket, it keeps working, just not in the fashion that other brains works. So you get some kind of disorder like schizophrenia or bipolar.
I was involved in a few projects where they were mapping schizophrenia with SNPs. Looking at the data, it's clear that some individuals cluster around certain SNPs...but most of them are scattered everywhere. Lots of things can cause schizophrenia, because it really isn't a single disease but a set of common symptoms.
I would be hesitant to assume that they're not genetic, though - I've personally decided not to have children entirely due to the fact that mental illness is so prevalent in my partner's family (about 50% of the current < 30 year olds in the family have either been hospitalized or tried to kill themselves, and I'm not willing to subject a child of mine to those odds). I frankly don't believe that this family has just been randomly unlucky, it seems far more likely that there's something genetic behind it, especially since even the "sane" people in the family tend to be so unhappy that they probably should have been treated for the same things that the "crazy" people have been hospitalized for.
DNA may only be one piece of the puzzle, but I think it's a big enough piece of that puzzle that we need to look into it a bit more. I can't recall the last truly "crazy" person I've interacted with that didn't have something messed up in their family history...
That said, epigenetics definitely could be the issue here, my partner is of Irish stock and that's one of the classic cases of epigenetic poisoning (the famines caused all sorts of epigenetic nastiness, a lot of which has been shown to persist for generations). I totally believe that some lineages have seriously detrimental baggage associated with them, and I believe that hers could be one of them.
In the end though, I don't know that there's much difference - a person's heritage, one way or another, ends up determining their children's susceptibility to mental illness, and if that susceptibility is high, a responsible "parent" should perhaps choose not to have the child.
Just my opinion. And to be fair, if everyone followed it and was a "responsible parent" (i.e. not having kids if your family history is fucking nutters), the woman I love would not have been born, so I don't hold it that dear...