My step-daughter is refusing to even speak to her mother let alone return to her or even visit her. Her mother texts her threatening messages the child is only 13. She is scared of her mother! What shall her dad and I do to not make her have to return because @ this point she is refusing?

The first thing you need to do is get her to explain why she doesn't want to go back to mom. You really can't decide what to do until you understand what is behind this. For example, maybe she did something bad like bothered her mother's favorite top and ruined it. Something like that can be dealt with fairly easily. On the other hand it could be that her mother's boyfriend tried to touch her. That is a more serious issue.

Don't prompt her with suggestions, but do get her to explain.

Once you understand the reason, if you feel she would be in danger to go back to mom, then her father needs to go to court tomorrow and apply for an emergency temporary custody order until a more thorough investigation can be done.