I would still like to know about the attitude and behavior of the sisters to their brother, there is more to this story than one child fighting his sisters...we are seeing only one side ...
I would sit his mom down and say if you don't listen to me I will call CPS. That should get her attention. And then if she doesn't listen to you, make the call. Raising children is really hard, especially when they have behavioral issues; but not impossible. Guess what, there are tons of programs out there to help you, not hinder you. It sounds like she needs help with this child. Talk to her about getting a PCA (public care attendant.) This person will come into your home and help care for the child. Both children need attention and care. It sounds like he needs some extra love. He also needs to be receiving weekly therapy. Mom isn't gonna want to do it, but if she ever wants her son to function in the real world she needs to.
I don't like the idea of removing children from their home because more often than not it makes the situation worse. In many states CPS will come in and make the parents start doing what they are supposed to. If the state is overly happy about taking children it's unfortunate. There is also respite care that she can send him to if she needs a break. Does he have a diagnosis? It will be much easier to get him this help if he has one.
Several years ago a mother and her autistic son lived below us. This child was out of control. He would try and hit my DD and run constantly. One day he was running on wet pavement while I was doing laundry. I got down to his level and said, firmly, "stop it, and sit down!" He dropped to the floor immediately. His mother said "how did you do that? I gave up on him a long time ago." I just stared at her in shock and said nothing because who would give up on their child?! I hate to say it, but it sounds like your sister gave up on her son and that is really sad.
I agree with everyone else! CPS needs to be called immediately for the sake of the children especially the girl. The boy needs severe therapy counselling and possibly needs to be removed from the home in order to get the proper help needed. Your family may dislike you for now, but eventually they will come to a realization that you did the right tyhing in getting them help for the whole family. This will have to be a chance you are willing to take. In a sense to me, this is a form of "tough love". Please don't allow this to continue for one more day cause eventually someone is going to end up severely injured or dead. This is just my opinion so if I have scared you, I am truly sorry.
Again, how old is the niece? And is she the only other sibling? I would tend to want to get her out of there so she doesn't have to take all of this.
I would also ask how do the sisters behave towards him older girls can be very challenging, I think that more goes on in the family and it isnt all about this child ,there may be jealousy going on and that would create a situation ...
I agree with the others that this is a very hard dynamic.
From your sister's perspective, having a child that no one in the family likes is VERY hard. I've seen this dynamic before, and parents often toss the other kids under the bus to keep from disciplining a child they have serious worries about. So they reconstruct everything - now, the difficult child is "normal", the others who upset the apple cart in defense are punished, and people who speak the truth in an effort to help (or in an effort just to tell the truth) are shunned or avoided.
I do think, though, if you call CPS that will be the very end of your relationship with that family. It's something to consider.
How is his behavior in school, and with his peers, do you know?
Very often the dynamics in the home are a problem there may be more to it ..definatly some help is needed .
I would call the CPS, for the sake of the niece. She is getting beaten by her brother so the family can have harmony? Forget it, that is a twisted game going on. How old is she?
I have not experienced this, but I have two thoughts that pop to mind. One is that this child has been allowed to get away with this behavior, and knows that the more he screams, yell and threatens, that others will give into it. So my first thought is that setting proper boundaries, and that others not catering to everything he wants and staying firm will do the trick.
The other thought is a little more frightening, and that this child has become violent and needs intensive therapy.
The problem is that this is your sisters child and not yours. I don't mean that disrespectfully - I think it's wonderful that you care enough to attempt to intervene. But I think its a difficult thing to do, and I think your sister has to see this for herself and get him the help he needs. Honestly..beyond telling your sister what your thoughts are (and that has to be done carefully and tactfully), I'm not certain much can be done. If you believe your niece to be in danger then you must call cps in order to protect her, but other then that, I dont' think there is much you can do beyond saying what you have said here. It's tough, and I empathize. I agree with what you are saying, but intervening in another families life is not an easy thing to do. Best of luck to all of you.