I don't know anything about the other boy. But there are reports that the foster parents had concerns about Connors sexualised behaviour.
I don't think you can blame police or sw the children were removed at 2 and 5. How would police / sw know what was happening in the house until Connor was able to say or showed signs of abuse?
But the sexual abuse must have been known about and is probably what led them to be taken into care in the first place.
Maybe there should have been better support therapies for both children when they were very young, maybe they would have been better separated, so many ifs and buts.