I've been in your shoes ( have two stepdaughters) and also worked with families like yours as a family therapist for years.
I don't think being a friend is a good idea at all. First, I think you need to talk this over with your fiance and ask for his support and backing. (One of the major issues my fiance and I had was that he felt guilty and was always giving in to his daughters, then would get over-angry and take everything out on them.) If you've got his support so that you talk over how you're going to approach an incident and then support each other when you're with the daughter, things will fall into line. If you let her play you off against each other, it won't be any bed of roses.
Second, you're going to get some flak from your stepdaughter, it's part of the package. She's angry and you're an available target. She wants things back the way they were, even though that's not reality. Don't take it personally. When my stepdaughter came to live with us, she swore at me constantly. (She'd been mistreated by her mother and was in sorry shape by the time she got to our house.) Things got so bad, I finally said either we all get into family therapy or I'm leaving. (At that point we weren't married yet.) We started family therapy sessions at the local family center. It was inexpensive, a sliding scale based on what we could pay, which helped a lot. After a few sessions, the therapist starting seeing our daughter and eventually, she came around. Later, we had to have sessions with us and the grandmother because she was interfering. We basically ignored the ex-wife, although she used to call at 3 a.m. sometimes and leave cryptic messages.
If you can't get your fiance to help, suggest a session or two with a family counselor. (Don't go to just anyone. Go to someone who specializes in working with families because they know how to work with family dynmaics.) They can help you get your feelings sorted out, make a plan and stick to it.
All best,
Carolyn
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