What I'm trying to understand is why she isn't ranting at her mother, given the fact that her mother, when drunk, does everything that triggers her. In the past she did scream at her mother, but she certainly hasn't bullied her mother like she did to me. Currently, as a rule of the house, her mother said she wouldn't tolerate that anymore. Ok, but I said the same, and it didn't work. And my word is much firmer than her mother's. Maybe it is the fact that she sees me as emotionally strong and "unshakeable," while her mother is seen as senior, vulnerable, and breakable. If it's not that, then I'm missing some piece of this puzzle.
I'm curious. You said she could come home for a day, and the day went pretty well. So you let her stay a second day and it all fell apart. I wonder if that alone explains what happened...she became so stressed the 2nd day over staying/leaving, it just all boiled over.
Your wife currently has emotional problems with you, so you'd see the brunt of her frustration. These boundaries are also pretty new and it's perfectly normal for things to get worse before they get better; she's pushing back because she's used to being able to. It's a transition period that nobody would handle well.
Why doesn't the mom receive the same treatment? Because that's not new and mom's boundaries are established. Also maybe because if mom kicks her out as well, there's nowhere left to go. BPDs always have someone they're close to, and even though mom isn't ideal, that's what she has right now so she's making the best of it. The relationship with mom lets her remain bitter with you...I'm guessing the opposite was true when you first got together.
It's also possible that your "firmness" could also be too strict for this transition. Your boundaries should be based on her decisions...if she can't stop screaming, she can't stay. That's fair and it's her choice. But if she didn't lose it on day one and she was going to be kicked out anyway, that could cause a good bit of stress for anyone.


