Welcome, TravelBug88!

Let me welcome you here to bpdfamily and wish for you as much help and support as I have received. It's clear you have a lot in common with many of us here. This is a community where we help each other, so I'm sure if you keep posting and reading you will find it helpful.
Thank you for sharing what you have thus far:
Her friends and coworkers call me asking me what they should do. I don't want to talk to them, I feel very pressured into spending time with my mother and be subjected to more abuse than I should not be.
I am so sorry you and your brother are going through this. This is a toughie, but amazingly, I've been through the very same thing that you are going through, except that my mother was diagnosed with Stage IV Hodgkin's Lymphoma rather than breast cancer. This was many, many years ago, but I remember well how crazy-making it all was.
First, although you may feel pressured, remember that you do not have to do anything in this world that you don't want to do. If her friends and coworkers are calling you, and it upsets you or puts you in a bad spot, do you think that you could just let their calls go to voicemail? Then, you could decide to return the calls at your leisure... .or not return them at all.
... .but she can't take care of herself, and being around her is not good for my mental well being or my brothers.
You mentioned that your mother has coworkers. Is she still working? Was she able to take care of herself
before the cancer diagnosis? If so, what has changed? Does your mother have friends that she can rely upon?
I understand that being around your mother is extremely tough on both you and your brother. Not to pressure you further, but, do you think that you would be interested in learning more ways to cope with your mother's behavior and possibly learn new strategies with setting boundaries in order assist you when interacting with her?
It sounds like you have really had a tough time, and I'm sorry for that. One thing that can really help with allaying some anxiety is understanding. And, it sounds like you are willing to understand BPD better, and that is pure genius, in my opinion. Luckily, you've found our website which can help you in that regard in spades. When you're ready, just start reading the site articles and do the Tools and Lessons located in the right panel of this page. The other thing that you'll find here is support for
yourself.
I believe you will be greatly comforted by the support here and the fact that we really understand what you are going through. We've all been there to varying degrees. Take care of yourself. We will look out for future posts from you.
Keep writing, keep processing, keep learning!
-Speck