I echo this response.

Merely having a disagreement does not make someone BPD. But, obviously, there is a great deal more to the story. We don't need the details but the context of the situation would help us greatly in how we are able to offer support.

I witnessed the go rounds between my mother and sister-in-law. Even though my SIL was in fact mentally ill the unreasonableness came from both sides.

If your daughter-in-law has in fact been diagnosed there are a number of things you might try to do. Most larger communities have support groups and resources for friends and loved ones of the mentally ill. If you can't find such sources of assistance locally, I am sure you can find it from the your national mental health association. It may be worth having a therapy session or two of your own to discuss how this is affecting you. You may, with their help, be able to formulate a plan. Where is your son in this dynamic? Remember too if it comes down to making a choice it is going to likely be with his wife.

I am glad you are not fogetting your own needs as this obviously affects your own mental health. Reaching out was a good thing to do for yourself.

And finally, if this BPD has in fact been diagnosed do what you can to educate yourself. But please, if this is not a diagnosed situation please do not try to do so yourself.