How can I bring my sister with her four kids into the United States from Serbia away from her abusive husband?

She has been married for 17 years with an abusive man. His nationality is Muslim-Albanian. Her nationality is Yugoslavian. Legally, they are divorced, but under the Serbian law, her husband can live in the house where she and her four kids live. There is no way she can look for another house to live in with her kids because she isn't financially able to. She and the kids are afraid of him. Three of the kids go to school, and they are excellent students. I saw them for the first time after 17 years, last year. Her husband is unemployed and doesn't want to work and she's the only one that works to support herself and her children. She provides him food and comfort because she is afraid of him. What's the first step I should look into doing in this case? And also, I don't have anybody in this country except for my husband, and two wonderful children. And, I wish my sister was next to me right now.