Most crimes like this are, sadly, committed by someone who has some connection to the family. It’s standard to investigate connections first. That’s not “appalling” to suggest, it’s just a sad reality of these crimes.
They should be focusing on everyone connected to the family if known. It would be negligent not to.
The confusion came from the way the article was written. They didn’t know the identity until afterward.
Something like 60-70% of violent crime involves victims and offenders that know each other, and with murder and sexual assault it's 70-80%.
I just want to point out that there’s a huge difference between thoroughly investigating the family after abuse of this magnitude has been proven, and making parents legally culpable for any harm that comes to their children in general.
We can react to the fact that mothers can do more to protect their children from abuse in many ways. We can give them better access to information and support in getting away from abusers. We can create better links between police and communities they serve. We can create more pathways for children to be exposed to healthy adult behavior and connections with healthy adults, even when the family is dysfunctional.
But when we find evidence that existing supports have failed, deeply investigating why is critical.