Hi caps4:

Don't apologize for posting. We're all here to share, support, and be supported. IT sounds like you're really struggling with wanting to have your sister in your life, and not being able to navigate the BPD cycles. I'm sorry that at this time of joy in your life, you're getting all of those hurtful emails.
There are a lot of tools here to help. You said you'd tried SET and it wasn't working well with her. Maybe starting with some boundaries about what you will and won't tolerate?
Boundaries Tools of Respect might be a good place to start.
Keep posting. It really does help to have someone to share with - the joy of your pregnancy and the struggles with your sister.
welcome.
ambi