Just tell maybe not social services but I know many people who work in social services and I'd just let them know to keep an eye on that family.
There is little you can do in that situation, I am an avid redditor and there was an AMA a while back of a guy who's mother abused him and his siblings, and many times his father confronted her and went to social services and the police but children are surprisingly loyal , him and his siblings were adamant that their mother never laid a finger on them despite her abuse, as she abused him and enforced propaganda that all men are rapists and women do not willingly partake in six he had trouble with girlfriends and physical relationships, he was still living in his mother's house at the age of 18 and his mother mustn't have wanted him there because she framed him to have sodomised his 3 or 4 year old sister, he was prosecuted but eventually found justice, there is little to be done except watch from afar but attempt to intervene if given the correct opportunity, be careful though, as you put yourself at risk by getting involved. That is all I can provide as advice, oh and to summise the ending of that story all those siblings are living happily and are on great terms with eachother i think they are currently prosecuting his mother, his father separated from his mother ling before the sodomizing and the mother had remarried , so things generally get better and work themself out. That was weirdly structured but I think you catch my drift