Hi! I'll be glad to be of help with this issue.
I can imagine how frustrating this situation must be for you. You are trying so hard to be fair and it's so difficult to watch how your husband just doesn't see that the parenting he's giving his daughter is not helping. It's not helping him integrate the situation of his having a wife and her having a step mom. It's not helping your relationship with him. And it's certainly not helping your relationship with her.
Parents of divorce have a lot of stress to deal with in parenting. And their guilt can overcome them and they become permissive and overly non-confrontational. This is more common than rare. Because in truth, the stress of parenting in divorce is overwhelming to many adult parents. And it doesn't go away as the children grow up. And so when one tries to talk with them, they very often just turn defensive.
The discussion, then, that you wish to have with her has two dimensions we have to consider: one is how your husband will back it up and the other is, first, how she will take the discussion. I want to focus on this part because it may make the second part go much better.
You want to present the following rules: "We will not tolerate in our presence or our home -name calling -confrontation: loud talk, argumentative tone -destruction of property -rude behavior or the like, i.e. condescending facial gestures".
They are all fair. I'd like to see you prioritize them into a hierarchical list as opposed to the way you are thinking of them: as items that everyone should respect. Here's what I mean:
Loud talk may be less serious a violation than name calling. Condescending facial gestures may be even less. The importance of such prioritizing is that it allows you to not just have one consequence for any type of violation: it's war or nothing kind of situation.
Remember, after this much time being unhappy with dad having another wife in the middle of her family life (this is most often how kids view second marriages), she's gotten used to a certain pattern with you. That pattern is indeed hostile and confrontational. But she at least knows the rules of that pattern and it has become predictable. So she keeps playing it over and over.
Your goal is to change that pattern, not to just punish or exclude. Therefore, decide if you want to have just one conversation about all the violations or separate ones starting with facial expressions.
Whichever way, the part of the discussion about her facial expressions needs to be presented as not so serious. It's your first effort at changing the dynamics between you two. So, you want to converse about it lightly, asking questions about whether she even recognizes any more what her expression is like when she's with you. And how you think she's so much prettier when she smiles and is friendly with people. Highlight how much more inviting and pretty she is when she smiles, etc.
Then, before the rules making, ask her if there's a way the two of you can signal each other if either of you is being not-smiling. So that you can have a great visit, or a nice visit. In other words, you're trying to see if you can get her to buy in to a different pattern.
After that attempt, you can begin to introduce the rules aspect of things. And by not equating looking nasty with calling names, it gives you a chance to have her at least not go "all the way into being that nasty step daughter she's gotten used to being.
Okay, I wish you the very best!
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