She is emotionally abusing her kids, your SO and you.
1/ Get a solicitor/lawyer now!
2/ Document EVERYTHING
3/ ONLY the BD should communicate with the BM
4/ All communication should be written - it’s evidence
5/ Send an email explaining the terms that were informally agreed and ask for it to be maintained
6/ Send an email stating that you ONLY want to hear about issues relating to the children, any other emails/texts/phone calls will be seen as harassment
7/ if the harassment continues, call the police. Communication about children is fine, comments and berating you/SO is not
8/ If she calls, voicemail it, no message, no response. The rules I use are email for all normal (not urgent comma), texts for important things that need a response quickly, phone call for absolute emergencies. Give examples in an email to her. Always respond to texts, if the voicemail isn’t urgent then respond with an email
9/ Set boundaries with her, you control the boundaries. She will learn them
10/ Read this book: https://www.amazon.co.uk/dp/B014W0587S/ref=cm_sw_r_cp_api_i_kR4CDbGZ9H4K9_nodl
11/ In ALL communication, keep it business like and do NOT stoop to her level, regardless of what she’s saying
12/ If she’s not civil - no response
This approach is slowly working for me. I’ve been through exactly the same thing. It’s got so bad that I’ve filed for residence of my children due to her emotional abuse of the children.
GTFO If you aren't married and have no kids; cut ALL communication That includes changing your phone number, blocking her on ALL social media, and filtering her emails to be deleted. If she changes her email or social media names and attempts contact DO NOT READ or INTERACT with her in ANY way. If you have mutual friends be VERY careful in giving your number to them (I suggest not even doing it.) You may be surprised to discover that mutual friends have seen the BPD abuse and will be very understanding. I was married and had a child with my BPD ex of 15 years. I am now almost 2 years out and my life is 1000x better but I still have issues to this day mainly due to the fact of having a child with her. It took over a year for her to get the message after I left.
This book is a MUST for you to read as it saved my life/sanity. (Have a friend buy it if she has access to your email or Amazon account) Say Goodbye to Crazy: How to Get Rid of His Crazy Ex and Restore Sanity to Your Life by Dr. Tara J. Palmatier, Paul Elam https://www.amazon.com/Say-Goodbye-Crazy-Restore-Sanity-ebook/dp/B014W0587S
This is a YouTube series done by the writers of the book... https://www.youtube.com/watch?v=M2KyHtFso0E&list=PLGL4PxshBNxBnIihTluJrJgvhbTQWAOAk
Also, you need watch Terrance Popp, he touches on a lot of things regarding women in a humorous but serious way. Go and look through all of his videos for the ones that pertain to your situation. https://www.youtube.com/user/redonkulaspopp/videos
If you have a child with here... I can not begin to explain the torture and heartache you will go through... but based on what your original post says you don't seem to have one KEEP IT THAT WAY and GET OUT!!!! Added comments... In the end it is up to you to get out she will NOT let you go until she has another to replace you and then will attempt to get you back when that fails (rinse and repeat). Believe it or not you are addicted to "CRAZY" (BPD woman) and only YOU can kick that addiction.