Do we know anything about the mental state of the 19 year old? Only asking because I worked in a residential treatment facility for severely abused children. There were 19 and 20 year olds there that functioned at the age of a 10 year old (or lower). I noticed that the males born crack-addicted were especially poorly off, even after being removed at birth. It was basically the place these children ended up when foster/adoptive families could not handle them over time.
We'd see all sorts of weird behaviors. Cutting, stealing food, hoarding food, making drugs, sticking fingers up places they shouldn't be stuck (for hours), attempted grooming of other children, false accusations of staff members (there is a reason we had one leg in the room, one leg outside whenever speaking to a child in the room). After working there for a year and a half, about 3 times the average staff could handle, I quit. The constant biting, kicking, punching, and spitting was a lot to go through on a daily basis. Sure, we had happy moments, but the children would also snap for very little reason, some due to brain damage from being beaten in the head previously. If I felt myself getting frustrated with a child I would reread their case file to regain perspective on what they had been through and why they were acting out.
It's quite possible the 19 year old is/was not able to function in a normal setting. I do think these children suffered abuse, but how much I am unsure of. I don't feel like the family would have run from CPS unless something serious was going on, CPS is usually tries hard to keep families together. It makes me wonder if one of the other 3 was already missing/dead, because stuff really hits the fan when you can't prove to CPS where the child is. Otherwise they likely just monitor you, send you to therapy, groups etc.