SORRY - THIS IS ANOTHER LONG ONE.
I agree with most of what everyone else has said here, except when people ask why the relative didn't do anything - I'd like to play devils advocate and say that maybe they did. SS's do not always act on reports.
In my case, I had severe concerns about my grandson. He and his parents moved out of my home when the baby was just over 2 1/2 months old. I told his parents that I was very concerned about the way his father treated him and would be passing my concerns on to SS. I called SS the day they left to move into their own home and gave a list of things that I had observed, dealt with and prevented during their time here. Two social workers came to the house to discuss what I had told them, it turned out that they knew grandsons father and said that they were very concerned about the baby.
Later that day - We were also told that they had visited the baby and his parents and because of their concerns would make twice weekly visits to ensure he was well and that his parents had any support needs met. I wasn't happy with this - but it was better than nothing.
Over the next nine weeks, DD would visit us almost daily, bringing the baby with her. Although he appeared healthy and lively, he wasn't growing. I asked my daughter on several occasions what her HV had said about baby's size - she produced her 'baby book' he had been to see the HV on regular occasions and she had repeatedly marked his weight and height on his growth centile charts - making it obvious that the baby had dropped by a centile over such a short period. Although I was obviously concerned, the HV refused to discuss anything with me, but simply said that the baby was healthy just small.
When baby was 5 months old - DD brought him to our house and his face was covered in bruises. His dad had been taking care of him the previous day as DD was ill - and claimed that baby had been laid on a cellular blanket resulting in the marks. I insisted on taking him to the hospital, pointing out that as soon as a doctor saw him, he would be referred to SS - DD flatly refused and got baby ready to go home. I told her that I had no choice but to report this to the SS. She went home and I called SS. I explained the situation and said that baby had a social worker who had been visiting and supporting the family for around nine weeks.
Turns out that the SS had actually "...lied to self and DH to shut us up." (their actual words). Even though they knew baby's father and his violent and abusive history, they assumed that we were "interfering grandparents" (again, their actual words) and left it at that. The baby had no social worker..they'd told his parents that he didn't need one.
Emergency duty officer went straight to house, after some refusal to let her see the baby - she got access and baby ended up being taken into hospital, turned out he also had a chest fracture and under the bruising on his face - fingerprints coule be seen quite clearly.
SO - it is possible that these people could have been reported to the SS at some point - the SS may not have been 'arsed' to do anything about it.