My thought was that perhaps this child either suffered some trauma or suffered from a mental or emotional issue that didn’t affect the other children. In some cases if parents do not want to commit a child a court can order a child to be placed, if it can be proven to be in the best interest or for the safety of the child. A recent case I am familiar with happened this way after the birth of a baby girl. The older boy (12) had lived at home despite severe emotional issues until the baby was born. But then it was ruled he could be a danger to the baby and placement was ordered. Not saying this is the case here but we just don’t know the circumstances.