D.D.
I am in a "blended" family. My son is from a previous marriage, and I have two daughters. I have been married to my current husband for 15 years, and my son is now 19. My husband never wanted to discipline my son, even though I wanted him to. He felt that it wasn't his place. During this time, I didn't know that his instincts were good. His relationship with my son has always been good.
Then, last year I was watching Dr. Phil on TV, that you can NOT discipline the child of a previous marriage. That's your HUSBAND's job. He has to be willing to step in on your behalf. You can be a friend to your daughter. You have no history with her is how Dr. Phil put it. Once you establish a few years together, that will probably be different. These kinds of problems are the major reason that 80% of second marriages fall apart, so you have to make sure that your husband teaches his daughter to respect and listen to you. My ex-husband attempted to discipline (punish) 3 children of the new marriage he had after me and is now divorced. They hate him. The marriage couldn't survive that. You have to tell her, "Yea, I'm not your mom, but in this house we have rules and I'm concerned about your welfare". Tell her you love her if you do. And have your fiance enforce the rules. This is a good time to test and see if the marriage between the two of you will work. If he teaches his daughter to respect you and follow the rules you both agreed to behind closed doors (without her present), and tells her what they are, then this relationship will flourish. Love and respect has to be all the way around. You to the children, him to you, you to him, the children to both of you.
When she comes back all defiant from the ex-wife, then talk to her and find out why she became angry. She is probably feeling safe enough with you to "vent" her frustration and anger with her mother, so that's why you're seeing bad behaviour when she comes back. Realize that it's not your fault, and that if you get her to talk about it, and tell her you will try to help her through it, but realize, since she's only 6, she may not have the ability to express her sadness and frustration with her mother. If you're right and her mother doesn't really care about her, that is VERY devastating to a child. They feel unlovable, rejected and abandoned if their own MOTHER doesn't love them. All you can do is love and support. Realize also that her mother may be telling her bad things about her dad, and that's also causing her bad behaviour because she got a dose of lack of respect when she was there, but she may not want to tell you that. These are not things you can punish. You can only love them away, and your husband has to deal with the discipline.
My mother LOVED to rant about my dad. I had to keep telling her for YEARS to stop.
God bless you for loving your stepdaughter the way you do, and I hope that your daughters "adopted" and otherwise (my husband NEVER called my son Stepson - just son) will become healthy and happy adults, and that you have a happy marriage.