OddOneOut! Welcome!
So sorry to hear about your family and I hope you have some support beyond these boards, but at least we all can understand where one another is coming from (sometimes those without some BPD experience do not). I'm sorry for your loss and the trouble you are having connecting with your brother and mother, I know it is especially hard when they find someone new.
Have you read
Understanding the Borderline Mother: Helping Her Children Transcend the Intense, Unpredictable, and Volatile Relationship?
www.amzn.com/0765703319I'm sure you've lived it, but it is super in depth and gives different descriptions of 4 types of borderline mothers you might recognize some of them in your mom. It also discusses children and those sections might offer you something you can relate to, and may give you some insight into your brother.
I would guess that to your mother if she is BPD and it was partly caused by the environment she grew up in that the abuse she is suffering from boyfriend (and past relationships as you mentioned) feels very homey to her. It's so sad to see someone we love accept this in their life knowing they deserve better. At first I thought BFF's "boyfriend" was going to be good for her since the way he spoke to her was how you would speak to anyone, instead of tiptoeing around her as so many in her life do. However, I realized that while what he says would work well with "normal" people for her it is actually invalidating. For instance she complains about work, and he says something like "You do it to yourself!" Or she gets mad and blows up and he lectures her "Why didn't you just tell me that upset you in the first place?" because he doesn't understand she is incapable of doing so without first learning the skills to do so. At first I was like this is great, maybe what she needs, then as I read a book on validation (
Loving Someone with Borderline Personality Disorder and I realized how horribly invalidating his words are. Sure he doesn't know that, and I don't think BFF understands the invalidation she received as a child contributing to her BPD and how that's exactly what "boyfriend" is doing now. To her it probably just feels like home. She already feels horrible for feeling how she does, then he's basically making her feel worse. I don't think it will work out in the long run because of this, and unfortunately I don't think he would learn to be empathetic to the BPD and learn to validate instead. I could be wrong, *cross my fingers* but I kind of doubt it.
I'm so glad you two found one another, it's so nice to have a hand to hold while you're on the wild ride. I hope you can both figure out how to get off... .another good one if you haven't already,
Stop Walking on Eggshells which is actually the first one I read.
Take care and I hope things get better!