Young stepmom has unruly stepdaughter!?

I am a young stepmom of a 12 yr old girl who has recently come to live with me and my husband (her dad) permanently. Her mom is 100% crazy trailer trash and a terrible mother who let her kids run wild with no boundaries or rules. The thing is, the mom has joined the military and washed her hands of her kids completely. And my husband is more like a friend than a father to this girl. I took her to the mall and she stole something from almost every store! She was looking at all the toys and stuff and I didn't know what to do! I'm afraid to tell her dad because then it will cause conflict between me and her. I really don't know what to do about this kid....I would never let her get away with half the crap she does if she were my real daughter.