Hey JREB,
The setting of boundaries is the common advice for someone who wishes to maintain a relationship with a person who has BPD. But it is not easy, so expect a challenge.
People who have BPD have a hyper-sensitivity to being hurt, being rejected and being abandoned. Not just physically, but emotionally. Something happened when they were very young. Could be a pre-disposition to BPD due to their emotional sensitivity, could be outright abuse. I do not know if your wife had a difficult childhood, but if so, that is part of the process of her forming BPD in the first place.
Now, forming boundaries can be perceived as criticism, which in turn is perceived as rejection. People with BPD feel on many levels that significant others invalidate them and their feelings. That what they are experiencing is not taken seriously and you are being dismissive. This has its genesis in some type of deep, core self-esteem injury. it can also be a projection of something that happened to her when she was young. She may have been emotional when she was young and her parents did not take this seriously, or rejected or hurt her in some way.
So when you set up rules (which are boundaries) she can get angry. When she gets in that place it can be quite a challenge for you to maintain those rules and boundaries. You are human and you care for her. If she starts attacking you, and verbally tearing you down, it can take its toll. You want peace, for you and the family. So, it is tempting to soften your stance. When you are on the receiving end of her anger, it is very hard to accept.
Plus, here is the most challenging part. If she breaks those boundaries you must be prepared to act. In other words there must be consequences. If not, she will not respect any of it and still she is going to push back. I can not tell you what those consequences should be, i.e. leaving the marriage etc. That is up to you. But she must be challenged, respectfully of course, to admit she needs help and must try to make the marriage work by pursuing that help.
Even if this happens, it is still going to be difficult. I wish I can tell you otherwise. Personality disorders are framed around mal-adaptive beliefs and behaviors regarding people and environment. Every day occurrences can be triggering mechanisms that send things off in harmful directions. And there is no way of knowing if and when that happens.
And people who have BPD are prone to always blame others for these things. One therapist actually coined BPD as a continual thought stream of "It is your fault." It is therefore difficult to have mature and meaningful conversations when the other party is convinced it is you who is doing this.
In the end it will take a lot of love, a lot of patience and a lot of understanding. And it is a cliché, but it will be one day at a time. She also needs to do her part. Go to therapy and practice what she is advised to do.