How can I get legal custody of my sister's children?

My sister lived with my mother before my mom had to be hospitalized and battled cancer. She is alive but due to my sister's inability to accept help or manage a household, they were evicted. She brought her eldest daughter to me just after Thanksgiving last year and the middle child in January. She refused to put the middle child in school near me and the child was just missing school as she and her boyfriend couch surfed. I had to enroll my niece in the HERO program in order to get her into school.The older daughter is out of control at 8. There is a long list of CPS cases where she has made all kinds of accusations and many have been unfounded. When I try to get the kids help, counseling, she refuses it. The kids do not have any health insurance because it was my moms job to keep such things going. I can not apply without proper documentation or custody. The mom has moved on to a new boyfriend but will not take back her kids and will not support me disciplining them.