Niece in custody of unmedicated bipolar mother?

My sister has not had physical custody of my niece for 3 years. The child has been living with my mother for a year and a half and has been thriving in school and at home. My sister was told by the child support division that they would soon start garnishing her wages to help support the child. Out of the blue last Thursday she showed up at my mothers and picked up my niece. My niece was hysterical because she did not want to go but the police said she had no choice because my mom had no paperwork giving her custody. My sister has been diagnosed bipolar and has admitted to being off of her meds. She has a long history of financial and mental instability including an involuntary 72hr hold in a psych ward. She owes student loans that she is happy to let her cosigners pay for and has told too many lies to count including that at one point she had stage 3 breast cancer. I'm wondering if there is anything my mother or I could do to get her back where we believe she belongs. We have called CPS and they assured us they would look into it but I have a feeling she will lie her way out of it like she does everything else. I don't think my sister would physically hurt her but the emotional rollarcoaster that is my sisters bipolar will have a lasting effect on my niece. She also lives with a loser boyfriend who has no job just smokes pot all day and she leaves the child with him while she works. I am at my wits end trying to think of ANYTHING I can do. Please help.