My Ex Girlfriend Stole My Sisters Identity Information

My sister just called me and told me that she was checking on her Equifax account and was having trouble getting verified. She learned that someone has accessed her account. And her social security number, date of birth, etc. is being used under my daughter’s name to the address where my Ex lives. I believe this was probably done to try to create credit cards using my sister’s information. Her bank accounts seem to be in order. Luckily she caught it in time. I’m trying to help my sister out the best that I can. I feel responsible for bringing this nutcase into my family’s lives. What can we do. Thank you