I think that for best results the younger sister and her child should probably move in with other relatives, preferably ones who can keep them for at least a few months.
You mentioned for instance her grandparents?
And I agree about, even if the child is in no immediate physical danger that you know of, that still doesn't mean that he is in no danger. He still could be.
And I can't believe these people (and I *literally don't* believe them) saying how "they are not abusive" while they are beating her up. 😠
(What???)
(We're such angry people and so angry at you that when you say we are abusive, it makes us want to beat you? What's *that* about?)
Anyway. It's ridiculous anyone (a) being so eager to defend themselves like that (to the person whom they are attacking) and (b) insisting that *they don't even do* the very thing that they are then in the act of doing.
Strange, strange people.
Sigh.
Your sister and her son should get away from them.
Please.
The sooner, the better, I say.