Hi - While I agree with you that rules and structure are necessary, you have to understand from your daughter's perspective. She has lost her mother, someone else has taken that place, her life has turned topsy turvy and if she is like any middle schooler, there are other issues at school she is dealing with. I think first you have to repair the personal relationship with her. She probably sees you as just the enforcer of rules and is missing the loving relationship she had with her own mother. Again, I am not trying to judge you and perhaps you are already trying the TLC first so excuse me for any misrepresentation. If all she hears is ranting and raving and nagging, 12 year olds dont respond well to that. Have you had any chance to spend 1 on 1 time doing mother-daughter fun stuff together? Tell her kindly and gently - that once a week, she needs to at least make an effort to clean the room and then reward her in some small way (whatever it is she is into). She will start to build more and more trust and likely to respond to you if she sees you not as a adversary but as someone she can bond with. If after all your efforts to bond with her, you still hear her screams and verbal abuse (and I am sure things will go back and forth), react to it calmly tell her you are trying very hard to develop a relationship with her and that you see her as your own daughter and that you cannot replace her mom but will try the best you can, But her words and screaming are not helping you and not acceptable behavior. Say it kindly but firmly. About cleaning bathrooms, I am actually in 2 minds about that. Frankly I think a 12 year old can do some basic wiping of the sink and floor but to expect them to scrub a toilet and bath tub, honestly, that can wait.
All I can say is - I have a 12 year old and by taking a very high handed and authoritative attitude, her Dad is loosing her. With me, I dont scream back when she screams. I try to be firm and kind, eventually she comes out, says sorry and does what I told her to do. A 12 year old is going thro a lot at school and if you havent read this book, 'how to talk to your kids so they will listen'., that is highly recommended. good luck to you.
Update: This is another book that is really really good "The Big Book of How to Say It Kids "
Also I was re-reading your post and you said she was 'snooping' in your bedroom and closet and you didnt take to this kindly. Personally I wont see it as snooping if my own 12 year old is in my closet but yes, if I find things missing, that is a concern, but it is usually because she wanted to try my new top as she felt she had nothing to wear. Is your bedroom off limits to her. When you caught her in the closet, you can ask if there was anything she needed from there and that you would be happy to let her borrow. Start out this way and then go from what reactions you get. I bet she just wanted to check out your clothes. As for the posters who advise calling police, that is the LAST resort..if you find her getting violent without control. But first try to repair the relationship, you may want to try group therapy between you , your husband and your step daughter. You need a good family therapist. I was just reading when kids are angry, it is usually they are very hurt inside and that is the only way they know to express it. Hope it works out for you. She is going to need lots and lots of patience and yes love and feeling of acceptance from you. And rules and chores will follow. Again hope the books help.