Girlfriends Stepdad Is A Pedophile But Never Got Caught. What Can We Do To Change This?

Throwaway but title is the main idea.
Some background: Girlfriend (19f) was sexually abused by her Ex-Stepdad for around 5 years. This was from the time she was 8-13. Contact was only orally and with hands as far as we know. She could have forgotten more due to trauma responses. I’ll spare the details but he did some fucked up shit.
Her mother’s divorce led to the stopping of the abuse. She came forward to her mother about her problems eventually and the mother basically said she didn’t care and to deal with it (spoiler: shes a piece of shit). GF had some dealings with a therapist and the police maybe 3 years ago but it was the typical “cant do much it happened too long ago. “
Where I think we might have some sort of way to catch the stepdad is that he has a daughter with their mother whom he is only allowed to see on weekends. She is around the same age that my girlfriend was when she was being abused (12). I’m not sure if she shows signs of being abused but we know at least shes definitely bullied for being ornery. She doesn’t talk about her father at all really. I don’t want to think shes being abused but it’s possible.
Edit: We live in Texas. Don’t know if this changes anything.
What can we do about this?
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