What to do when taking care of your sister kids?

Well im a mothers of two boys .my oldest is 22 my youngest is 11 Well i had my sister son every since 09 he left from home When she Try to beat him with a stick that had a nail in it so he couldnt deal with the abuse no more so he came to stay with me at the time he was 14 and now she doing.the same thing with my niece she only 15 yrs old one nite my SIS came home and couldnt find her keys to her house .so my niece know how het mother is so she was trying to avoid het mother so she goes into the kichen to Washington the dishes so she was like Why u got my water running she said mom im trying to was the dishes so she put my niece out at 12 at nite .Well while she was on het way to my house she said that all kinds of mans was trying to pick her (oboy i was fired up) my sister receive ssi for her kids even herself . Now i have no problem taking care of them but every since i had her son from day one she done nothing at all for him .even on Xmas-bdays which is mess up because while she looking good her kids look like **** .so she thinks that all her kids is going to stay with me while she steels receive the benefits .now my niece school calles cps on her so they came to my house to c my niece to ask her somethings about what happen to her the nite she got put out .so from then on they was shock to find out that i already have her son hes now 16 going on 17.so they told me to go to ssi and file for the kids .i feel bad because she thinks that she has the right of way to keep the kids benefits .what to do please help .