I'm sorry, but I don't think there's a way for you to avoid the rages. Especially if they happen for no apparent reason at times. Your brother might be able to get better at riding out his anger instead of taking it out on people around him, but that would take at least a year of intense therapy (based on my personal experience).

The only thing you can change is how you respond to his anger. Leaving calmly is an entirely appropriate response. Telling him you love him but you won't deal with him when he behaves that way, making clear you're happy to spend time with him once he calms down, those types of things can help.

I'm not sure what to say about your mother, though. Is she able to leave the premises when he acts out like that?

There is a book or a website with a title like "stop walking on eggshells" that supposedly tells people how to deal with people who have BPD. DO NOT READ this website, it's got extremely negative views of people with BPD and essentially sees them as toxic people.

The book "I act crazy sometimes" has a much more balanced view of people with BPD and gives tips on how to deal with someone with BPD at the end of each chapter. I think you might find it useful.