Welcome A Pandy: I'm sorry about the situation with your brother's girlfriend. How old are they? Unfortunately, you can't change anyone else, only the way you react and interact. It is probably wise to accept that you can't steer your brother away from the relationship (as you were advised). It is understandable that you would want the best for your brother and a life with a BPD can be a difficult one (and difficult for the entire family)
Is his girlfriend in therapy or getting some form of treatment for her BPD?
Does your brother understand the life-long issues he will have to deal with, with a person with BPD? Does he understand that there tends to be a genetic component to passing on some form of mental illness to children? BPD can be passed on either genetically or environmentally, or a combination of both. Most people with BPD have other mental health issues: depression, anxiety, bipolar, ADD/ADHD, etc. (which tend to be genetic).
The best thing you can do is to set some boundaries and employ some strategic communication skills. There are some good links to helpful information to the upper right of this post. The links below lead to some specific information that will likely be helpful:
DRAMA TRIANGLESETTING BOUNDARIESSET - COMMUNICATING WITH SUPPORT, EMPATHY AND TRUTH The drama triangle might be a good place to start reading. The Caring Triangle, towards the end of the article, should be the strategy to aim for. Take a look at it and let us know what you think. There are a lot of helpful people her, and it can help to post about employing some of the strategies and gaining the input of others and their experiences.