Social workers are human, too (meaning fallible). The social worker who lives opposite me, left her then young, teenage children (still at school) at home, alone for two weeks about eleven years ago and the food ran out at about day ten. They knocked on my then neighbour's door (who went to their school and was a few years older) for food, that's how we found out.
So do not take the fact that your mother was in CP "for years" as any kind of salve. She is digging her head in the sand by virtue of her ignoring you. That in and of itself is too defensive an action. She in all of her "wisdom" has allowed herself to be groomed. She has no use now. It's on you, and you do not need any permission to call the SS.
Abuse prevails because of self interest. "Self Interest" ranges from the very human emotions of "needs" and "wants" to " anything for a quite life", "it's none of my business" to plain old selfishness and materialism and straight up abuse.
I know that you do not want to fall out with your mother, but you already have. Can you not see that your sister's boyfriend's manipulation is working and in full effect? No abuser on God's Green Earth doesn't thrive off of confusion and disorder. But you have to ignore that , or else he has won. You have no need to fall into the "selfish" end of the "self interest" spectrum... not when you are the only adult in this situation who can advocate on behalf of your nephew.
Phone the police and the SS. They may be rubbish, too (as is my experience), but you have to be able to look your nephew in the eye. Abuse victims often hate the double betrayal of nobody standing up for them as much as the actual abuse itself.