I grew up with a mother who was mentally ill and suffering throughout my childhood, She has never sought treatment. Like that 12 year old girl. I turned out ok and she will too. I also have BPD and recent studies have proposed that BPD may have a hereditary component in addition to environmental factors. I would recommend that you take time to encourage, compliment, and show that child what it feels like to have someone who loves them unconditionally. Show her that she can depend on you. In my opinion, nothing helps a borderline like unconditional love and support. Good Luck to both of you and thank you for caring about this child. I wish I had had someone on my side when I was 12, maybe I wouldn't need so much support now.