I have heard that it has a tendency to run in families. Because it is more commonly dx'd in females it has a tendency to run down through the female line.

How much of it is nature (genetic) and how much of it is environmental (nurture) is hard to say... It is a combination of both...

I think that precisely what it is that has a tendency to run down is...

- Emotional intensity (distress / dysregulation)
That has a significant genetic component. But also:
- Mentalization (ability to say what you are thinking, feeling, wanting, needing etc and ability to attribute appropriate thoughts, feelings, desires etc to other people).
That has a significant environmental component.
And in conjunction... They both feed each other (harder to mentalize when you are distressed and harder to regulate emotions without the ability to mentalize)

I have mixed feelings about the BPD dx.
On the one hand it can be a ****** because of stigma.
Fortunately the situation there is improving, though it still has a long way to go...
On the other hand it can be useful because it can mean that one has access to varieties of treatment that are more likely to help.

Hang in there...