HEY VERONICA LODGE:I'm sorry about the situation with your MIL. I have a uBPD sister, so I have a little taste of your type of pain.
My husband and I are in the "all bad" category again along with my father in law and I suspect it's because my father in law confides in us and also the fact that my husband and I are extremely independent (and don't fulfil her needs).
You probably won't be able to change the pattern, unless MIL decides she has a problem to work on.
I'm sure you and your husband are holding FIL's confidences to yourselves. As long as you don't triangulate between MIL and FIL, you are perhaps acting as a listening ear of a therapist for FIL.
Have FIL or MIL had any therapy (singularly or together). Has anyone in the family received treatment for any mental illness?
Her weekly texts have dropped off so that I am the only one texting her just to stay in touch (and keep the peace).
How does your "staying in touch" with her keep the peace? Is texting the best means for this? If she doesn't answer, why continue? Perhaps she gets satisfaction in you sending her texts that she can ignore?
I think texting has it's purpose, but if used as a main means of communication, it offers a lot of opportunities for misunderstanding, misinterpretation and unrealistic expectations. Sometime, it might be advantageous to go "old fashioned" with a pwBPD. A little handwritten note by mail, might be something to try. Just limit it to brief pleasantries & don't give her any fuel for her fire. (i.e. a thank you note if she hosted something, a brief note to say hi and hope all is well, etc.) As HappyChappy mentioned, Medium Chill is a tactic to try.
Unfortunately, you aren't ever going to change her, only the way you and your husband interact and react. Best that both you and your husband are a united force in what boundaries you set and how you handle MIL. You can set a boundary to not listen to The
BIFF RESPONSE is another tool that you might find useful.