Sorry this is the situation.
12 is very young to be acting that way, and what complete dirtbags are these older men that would talk/entertain/google at her.
What you and your husband need to do is be very active about seeking custody of your step daughter.
My fear is that she has been sexually abused somehow already, OR she has NO limits on phone, tv, internet, at her Mom's house and is exposed to sex, sex, and more sex in this sex obsessed media of ours.
She needs to get away from the situation at home, live with you and your husband who will give her attention, set boundaries, and role model right from wrong the way her mother is not.
She also needs to talk to a child psychologist.
I wouldn't recommend a school councelor because that can backfire in giving her a rep at school.
I would also make sure that she dresses appropriately.
I would get her into something-- some kind of lesson or sport where she can take pride in her accomplishment and not her 12 year old warped sense of sexuality.
Best wishes for you....