J.,
I can relate. I have a daughter who is 10 (almost 11) going on 20. After her father and I divorced 8 years she lived with him for 5 years. She returned to me 1 year ago. He lives in Colville WA and I am in Kent, WA. 6 hours away. She is very manipulative and mouthy. She disobeyed me over and over. A child whether it be a natural or step child will have behavioral issues if they are not raised with your morals and rules for the first part of their lives.
First, you taking away her room privledges is not fair for the other child. In my experience my daughter would get verbally mean to the other child she shared a room with because she knew it would irritate me. Try taking things away that she loves. With my daughter it was her friends and golf. (Yes she golfs) Second, telling her to find another home is never good for the childs sense of security. I understand it was in the heat of anger, but it still hurts them a lot and hard for them to recover from it. Packing her belongings and telling her to leave you are sinking to her level. I am not attacking you because let me tell you I have been there. My 10 yr old physically attacked my husband who is her step-father who also defends her from me when I am exhausted. She wants to move back to her father house which is not an option. She was being mean to her siblings, and me. Once she saw that was not working she turned her anger to my husband. Well he called me and told me what happened and I had all day to think of the perfect punishment for her.
Here are a few ideas that have worked wonders over the last few weeks. Yes I said weeks. Grounding her from friends, phone, TV, movies, family outings (get a sitter), make her clean. I have made my daughter after her last incident with my husband clean the kitchen, living room, bathrooms (2), fold laundry and keep her room spotless. She is not allowed to play with her friends, use the phone, watch tv, etc...I do not allow her to use the dishwasher while doing dishes. It is all manual. If there is a plate, glass silverware that is dirty then I pull out all the plates (if it was the plate that was dirty) and make her rewash all. She has improved tremendously. She wrote him a letter saying sorry for her actions and mean words. She has been doing this for a week now and has a week to go. Yes I did it for 2 weeks. You have to be the parent and let them know that they are not in control. I felt mean but I had tried everything else. Since she had done this for a week with out complaining I did give her one day with her friends (today) after she finished all the housework and I inspected it. She was thankful.
I know you are in a harder position then I, but it sounds like our daughters are rebelling the same. If you need to talk or vent let me know. ____@____.com or http://blog.myspace.com/katrinastark. Trust me I need to vent sometimes and I talk with her stepmother often. Good luck. I hope to hear from you.