Divorce Advice for Men | Fathers Rights Divorce | Child Custody

My 3 girls (17, 15, 11) have not lived with their mother (80% physical custody, 50% legal custody parent) who's been unemployed approx. five & half of the 8+ years since our divorce, and in & out of alcohol & drug rehab, to no avail. They have lived with my ex-mother in law (she's very well off) as a compromise after the girls finally refused to return home after visiting me one weekend approx. two & half years ago. Their mother has been on assistance, but I refused to continue paying child support to her (as she only feeds her habits, not the kids). She uses her mother's address as her own but has not lived there, but I believe her mother pays her phone & utilities at the 2 other residences she has had during this time. The state calls me to collect. I've been to their offices with her true addresses, but they don't listen. I am sure she is entangled in some sort of welfare fraud and the girls will testify as to "who live where, and during what months and years" but they don't want her to go to jail. I haven't generated much income at all in the last 2-3 years but now I am starting a good job in Colorado soon (2 wks) and don't want this to linger. What should I do/consider with the limited resources I have for the time being?