Hi MrSandman,
First, it's good that you are there to provide a safe place for this girl. Growing up with a mother with depression and BPD, I had a similar place to go, and it saved me in many ways. 30 years later, the boy in that family is still my best friend.
I get the feeling of betraying her mom. It could also be that she fears the unknown, and this is a very valid issue. My mom asked me over 20 years later why I didn't tell the CPS investigator what was really going on. Maybe she had been wondering for years why I didn't, who knows? All I could think of was that as much as I wanted to be rescued at the time, I didn't relish going into foster care. I was 13, and looked towards my 18th birthday when I could escape, and I did. I can only guess her feelings, but that was my experience.
CPS agencies usually have an anonymous hot line. Maybe you can start there. It sounds like the counselor may be unable to do much if the girl won't open up. It's a valid fear. Many people can hide abuse. In domestic violence situatiins, victims often refuse to press charges. It can be complicated.
Other than doing what you're doing, you can learn to validate her feelings. This can go a long way. Your daughter is old enough to learn this, too. This may help:
Communication using validation. What it is; how to do itTurkish