I remember I also wanted to do something similar.
I actually talked to my therapist after regarding this topic.
Before that I even found some online materials that said that this is codependency trait. My personal opinion is that our codependency is built on narcisstic traits but that is for sone other topic.
You are still feeling responsible for her, you are still in 'white knight' mode... .
First, her family knows that she has issues. They maybe don't know details and maybe never heard about BPD like you did but they have some insight for sure. If they are close, if not - what is the point of telling them?
Also, considering the fact that they know her much longer than you and they didn't react/change/involve - again, what is the point of telling them.
And the most important thing:
Tou can tell to every person on this planet it won't matter and it won't change anything... .Only she can make the change... .You are helpless in your attempts to change someone... .