I've first hand knowledge of how problematic the extended family of child abusers can be. But the argument from CPS and the courts is that all else being equal

I've first hand knowledge of how problematic the extended family of child abusers can be. But the argument from CPS and the courts is that all else being equal (including probability of abuse in the new digs), the familiarity of grandparents, aunts/uncles, adult siblings is better for the child than a foster home or ersatz orphanage. I'm not sure if there's data to back that up, but given how frequently this occurs as an informal arrangement (I'm currently watching this particular arrangement unfold with my parents-in-law and a niece, and the particular details of that situation are, as you say, pretty bleak), I suspect that it is, on balance, the best outcome for a shitty situation. For myself, for my niece, I have to believe that doing the social support equivalent of taking them out by the woodshed as soon as abuse occurs is giving up way too soon.