Boymom528, first of all, congrats on your second pregnancy! Here's to a stress-free delivery in April. I know your little one will be thrilled to be a big brother!

Thank you for sharing your story. My perspective is a little different as I'm in your husband's shoes, my MIL has BPD. I'm also fairly new to BPD, so others here will set me straight if I'm way off, but my understanding is that BPD's use of silent treatment is driven by fear and a need for validation. Like any of us, pwBPD use tools that have worked before to elicit responses/meet needs. This particular tool happens to create major issues in relationships.
Do I let there be a long period of silence until she reaches out to me?
How long does she go without talking to you? My husband didn't call his mom for 5 days when we went on a vacation. He calls his mom every day, so this was a huge deal for him. I text my mom a few times a week, call her about once a month. Every person has a different setting. At what point do you start feeling the guilt set in?
How do I set clear boundaries that let her know she cannot treat me or my family this way but also maintain some sort of healthy relationship with her?
I know you already know this, but if she wants to offer the silent treatment, you really can't stop her. You do, of course, have control over how you respond and boundaries may shift the dynamics in your relationship. They define boundaries here as "the life skill of openly communicating, asserting, and defending
personal values." Can you identify some of your personal values? (If you check out the "Tools" tab at the top of this site and click on the boundaries article, you'll find more guidance about this.)
I know you're busy with life and kids. My kids are teens now but I remember the baby years well. I'd love to hear from you when you find the time. It's important to us that we move at your pace, so don't hesitate to communicate where you are and how we can best help.

pj