A.J.
Wow! I thought I was the only person in the whole world that went through this problem. My husband and I were married two and a half years ago. He has a daughter from a previous marriage. She was 12 when we got married. She has absolutely no discipline either. Her mother has taken the Gilmore Girls best friend approach and given all discipline up. And my husband had the same problem for a very long time. I looked at the situation and decided that I was the only person that would discipline her and proceeded to do so. That was about the WORST thing I could have done. Not only did it make my step daughter hate me, but it made my in-laws upset with me and put my marriage on the line more than once (about 10 times)
After two and a half years of being upset and fights and hurt feelings on both sides I think I may have figured it out. As much as I want to discipline her I just can't. Not only does it make me the evil step mother but it actually back fires. Because it's me that did it, she did the exact opposite and because her parents saw that it was me doing it as well, they didn't enforce the discipline. So everytime my step daughter came over I dreaded it. It was horrible.
So after my long story, what advice do I have? I have learned that she is not my daughter, but somehow she's still one of my kids. I have no right really to discipline her. DO NOT discipline. It has to come from your fiance'. Be your step son's friend, in an adult way. I have worked very hard though to have a strong friendship with her. It's tough though. Sometimes she's just nasty for no reason. I have had to learn to become the adult in the situation. The thing that has amazed me though is I have still been able to be a strong force in her life. I have been able to talk to her and help her in ways that her parents have not. We've been able to talk about drugs, and sex, and alcohol and even how she feels about her parents. I've been able to show her that her dad loves her more than she realized. The thing that I learned is she actually craves the discipline, but it can't come from me. The only discipline I offer is reasoning as far as some of her faults. I kind of offer consequences that could happen outside of her discipline. As far as her grades (which are horrible) I show her college acceptance requirements. I explained that they would be looking at her grades and she would not have a chance to sit down and show them how So smart she really is. I explained that it was all based on forms.
I have helped my husband try and see the behavior that I saw and helped him try and understand how to deal with it. He really didn't understand what I saw with my "outside eyes" or how to deal with it. So a little help from behind the scenes has taken care of the problems. And the other thing is look for the positive. I saw so much negative in my step daughter that I could hardly be in the same room with her. But when I started looking for her positive traits I began to love her as my own.
I also suggest family therapy. But generally I think it should be with the mother, the step son/daughter, and the father. I think that if there is a discipline problem and tantrums there's probably a much deeper issue. If it was a nasty divorce or there is nastiness between them the children pick up on that. I think it's very important for both parents to be in contact. No matter how good the children are they will still play the parents against each other. So it's best to work out those issues asap.
Also, never say anything negative about his mother, in front of him. No matter your feelings towards her. I really can't stand my husband's ex wife, more for her mothering attitudes, than for the reason that he's my husband's ex wife. However, I have nothing but praise for her in front of my step daughter. Even when she asks if I don't like her. It's tough, but no matter her feelings for her mother at the moment she is incredibly protective of her parents to the other.
This is how I have dealt with my particular family. But each family is different. I was recently cleaning out my bookshelves and I laughed at the step mom books that I had bought. I'm sure their advice worked for their family, but not for mine. I hope this helps.