Hang on. Social services don't just take children into care because the parents ask them to, and quite rightly. If she had approached SS for help she may have received it (depending on her mental health/state of the house etc, we don't know how good/bad it was) but they would not have taken the child into care, unless it was terrible.
It is not against the law to ask family members to look after your child. However it is against the law to ask family members to look after your child for a prolonged period of time and not to inform SS of the arrangement. It's called private fostering.
It sounds like the mother was completely emotionally neglectful in that she didn't have regular contact with her son during this time, and during the contact she did have, failed to notice his injuries. However she is not at fault (unless she knew they were abusing him) or responsible for their actions. The woman was a family member. If it had been her mother who had been looking after the child would anyone be calling for her to be prosecuted? No, because most of us let our mothers look after our DCs for periods of time.
These people were cruel, sadistic abusers, but not strangers to her. As long as she didn't know what they were doing, she is not responsible for their actions.
However I would have expected her to be more interested in her son's welfare generally.