What can I do about my Ex-sister in-law using my families names to obtain instant credit and stealing her daughters #SS?

My ex sister in-law was convicted of identity theft in 2004, and given probation.
She has been using her oldest daughters age (20) social security since she was 12 and leaving a $2000.00 phone bill along with many other smaller bills. Now my niece has no credit and unable to obtain a credit report for the detailed information. While my brother was in Bagdad, his ex kicked out the same daughter, and continued to collect child support; none of it going to the daughter who was going to High school and got pregnant the last six months of her senior year. She graduated, went to medical assisting school, graduated and has a job, but when she applied for credit, it was denied. The ex sister in law has also used all of our families names and addresses to obtain instant credit. What can we do?