How do you deal with a difficult stepdaughter?

How do you cope with living in a household with a 16 year-old stepdaughter (well, not technically, her father and I aren't married) who disrespects you,will not take any direction from you without an attitude, undermines your authority, and constantly degrades and disrespects her father in your presence?

This behaviour happens daily and causes an enormous amount of stress in the household as well as on my relationship with her father.  We sued for custody last November, on my suggestion because of her constant fighting with her mom.  She also manipulated us into thinking that her mother wasn't providing well enough for her and was moving her around from place to place.  Our full intention was to give her love, structure, a less stressful environment, and help her make it through life.  Now she is fighting with us over the same things that she fought over with her mother.  I have come to the conclusion that she has a problem with authority PERIOD.

This girl came from a very unstructured household.  There was little or no parental supervision, mother was more interested in her boyfriends than her kids, mother was in debt up to her neck because of carelessness, etc.  She (the girl) has always been very manipulative, selfish, self-centred, and inconsiderate.  Her mother gave her no consequences for having 100 "tardies" in her first hour of class because she would not get up and be ready to go to school.  Her teacher flunked her for her lack of responsibility and the rest of her grades were D's and E's but she always had an excuse.  Her father could do nothing about this because the child did not live here at the time.

She would call us at least once a week and cry and complain about her mom not buying her, or some other drama and she'd complain about their fights.  After we learned that the mother and her two daughters were being evicted from their home, we sued for custody of the younger one.  The older daughter was 18 and chose to stay with her mother (because she didn't like to follow any of Dad's rules).  The first couple of months were pretty good, but after we stopped buying all the things we thought she needed and the household rules were put into place, it all changed.  Her father is a very responsible and good father and has standards and rules but nothing that any other ordinary parent wouldn't have.

She stays up in her room most of the time...

She stays up in her room most of the time and only comes down to eat, ask her Dad for something, or ask if she can go somewhere.  She talks crap about her father to everyone and is constantly degrading him to others.  Most of her friends won't even come to our house because of how she runs her mouth off about him.  Since she has been here, her attendance is pretty good and her grades have improved.  She is given £80.00 a month to buy her personal needs (hair supplies,deodorant,etc.) and does NO chores around here other than a few dishes every other night.  She has a beautiful bedroom and bathroom of her own and all the comforts that go along with it, she has very little respect for these things either.  She frequently lies about various things and I think that she is having a problem adjusting to structure, rules and authority.  Here, there are two sets of eyes on her, and most of the time it's me that is here with her.  Her father works full time and I am a full time college student.  She gets away with nothing from me, I don't tolerate any of her shenanigans.

How can we let her know that we are NOT here to make her life miserable, but at the same time her attitude and disrespect will no longer be tolerated.  We both want the best for her life but she is making ours so much more stressful than it has to be.  Does anyone have any helpful, constructive suggestions as to how to deal with her?

By: Hopeless?