Hi Moosemom:
The link below will take you to the Psychology Today, "Find a Therapist" page. Once you enter a zip code (perhaps for yourself or your brother), you will be taken to a secondary page. If you look to the left, on the 2nd page, you will find various categories to flag. Towards the end of the page, there is an option to select video counseling. Under "issues", you can click the "more" option and see a longer list that included BPD. It might be strategic to find a video counselor who is close to one of you (perhaps the one most apt to seek an office visit at some point)
https://www.psychologytoday.com/us/therapistsI was warned by a therapist that group sessions tend to NOT be successful, as generally one person typically feels picked on and quits participating. In your case, you might want to start out with just a joint session with you and your brother. You can't change your mom, but you can change how you interact and react to her and set boundaries.
There could be value with you and your brother working together on using some skills and boundaries with your mom. BPD Family has a lot of good workshops and tutorials for learning communication skills and strategies. It could be a good strategy for you and your brother to study some target subjects before a therapy session and discuss/reinforce some of the strategies during video counseling (with real examples of your mom's behaviors).
Just thinking that you and your brother could benefit from having your own joint sessions first. Taking an opportunity to vent about your mom with a professional, and learning to manage your own actions, reactions and boundaries, might be a good first step. It would be good for your and your brother to work through some thing before trying to get your mom in a session with both of you.
Perhaps, you can interview a couple video therapists and see what they suggest. Maybe you get in a few sessions, try some strategies and start to set boundaries first. Then, bring mom into a session with you and discuss specifics.
Generally, therapy doesn't go well, when someone doesn't want to participate. Starting out with the 3 of you might be a tough situation.