My 12 year old niece is being mentally and emotionally abused by her mother and step father. She ran away and came to my house in which her mother is constantly coming here and having her in tears. She is telling her she has to come home. Can I do something ?

There is little you can do if you aside from sitting down with her parents and having a good long talk (about having her stay with you)? The other choice of course is reporting this to child services, something you need to think twice about.
We (and possibly you too) can't possibly know the true nature and extent of the verbal abuse, and verbal abuse isn't considered actionable by social agencies. She may be being treated horribly and she may be exaggerating, as is common. She may have a lot of resentment about having a stepfather. If you care to try to summarize what she is reporting to you, we might be able to suggest something.