I have a nephew who lives with drug addict parents, my brother has never had contact with my nephew and doesn't want to know, I've never had contact either but I know he is being bullied and not looked after properly, social services and the police are aware of his family situation. Would there be anything I could do to help him? I feel so sorry for him and his little sisters

We first need to know what state you are in or if outside of the U.S. what country. Second, if you have no contact with them and your brother (the bio father I presume) has no contact with the family how do you know the family situation? You have no legal standing as far as removing the child(ren) yourself. However, you can make yourself/your home available to the case worker dealing if they are looking to remove the children. I can't really tell you what kind of odds you would have of having him and/or his sisters move in with you because of the lack of connection. I have known of children being placed with a grandparent or aunt/uncle but the children already had a relationship with that family member. At this point you are a stranger to the children (not placing any blame just not sugar coating it either) but so would another foster family be most likely.

What I would recommend with out knowing anymore about your situation is to see if your brother would go to court to fight for his son (and girls if they are his as well). Then if that idea doesn't go over well let the case worker know you would like to be considered if/when they do place the children. They can tell you any specifics of what you would need to do for your exact area for you/your home to be a possibility.

You need to be more clear. You hinted that your brother is involved but left us guessing.
Also, what type of help are you thinking about? We can't read your mind here.
Two lines of explanation for a complex situation like this, doesn't do the job.

My mother has had contact with him once every fortnight or so, it's through her I know things, but I feel if he had other family members in his life it would benefit him somehow, his sisters are half sisters and not related to me. His father is not interested in helping him, he's witnessed his mum getting beat up he sleeps on a couch they have parties all the time, his clothes are dirty and too small. He's bullied at school, he has such a miserable life

Was it ever established that your brother is the father of the boy? Still trying to read between the lines.
If your brother is the legal father, your family may have a chance of getting custody of the children (all three so as not to separate the children form each other), depending on laws and procedures in your jurisdiction.
Sounds like there are two other families involved too (the families of the two "addicts").
I guess that your mother could report drug activity if she witnesses it, but that can be dangerous for your mother. Some police agencies have anonymous phone lines to report drug activity.
Are you financially and physically capable of taking custody of the children? Do you have space in your home for them? But, if your brother is the father and he wants nothing to do with any of the children (still guessing because you aren't telling), there is basically nothing you personally can or should do except possibly report drug activity that you personally witness.

Is your brother their legal father? That makes a difference. But if he isn't wanting to do anything what I said earlier doesn't really change. If you are financially stable and have the room you can make your home available to the case worker for the children (I realize only one is related to you but it is harder for siblings to be broken up). Otherwise, any drug use, abuse, other dangerous situations that you or your mom witness can be reported but it sounds like that has already been done.