I can't see this as a good "therapy" for someone with BPD...in fact I think it's the opposite of what someone with BPD needs since most individuals with BPD struggle with abandonment issues. It's kind of like that old school way of thinking about fears-well let's see you are afraid to swim in the ocean...I know what will cure you of that-I will push you off a boat and just expose you to it. Uh..no that isn't the way.
I can see loved ones of someone with BPD setting up boundaries to protect themselves from the drama and "neediness" of someone with BPD. Again, we are talking about some people with BPD...not all. Some can be draining and I think finally get to the point where they may even tell themselves that "it's for your own good" when really it's for their own good (which is okay if they need to distance themselves, they should just be honest about it).
If both you and your therapist do not believe you have BPD-I think that is more to go on then what your in-laws say. The thing with BPD is that it's very complex and has 9 different criteria for which you only 5 to be diagnosed. With that combination of 9 criteria BPD can present in so many ways. My psychiatrist said I probably went un-diagnosed for years because I am not a drama queen and I am nice and respectful-I tend to never have huge fights or outbursts with other people and I am not manipulative. But a lot of the other criteria are there. I think BPD is one of the worst stigmatized and poorly understood mental illnesses. Though I have come across people that I am sure have it-I would never say "you have BPD" to them. If they were suffering I might suggest they talk to a psychiatrist about the possibility of it, but since I did not go to med school, and specialize in psychiatry don't think I am the proper one to diagnose them-nor are your in-laws.
Maybe this is a good thing-if they are not the greatest people to be around, maybe take it as a blessing.
Misfit