Hi OpalGirl 
I would like to give you a warm welcome to our online community  You are having a hard time right now as a result of what you've been through and are still going through with your mother. Being the child of a BPD parent can be very difficult indeed. BPD is quite a challenging disorder so I understand why you are having such a hard time. Has your mother been officially diagnosed as BPD?
The way your mother has treated you seems to have negatively influenced your self-esteem and how you view yourself. Something that might help you is to keep in mind that no matter what your mother says or does to you, her words and actions most likely don't have anything to do with who you really are at all. Odds are that her behavior is only a reflection of her own inner turmoil and negativity which she is projecting onto you. We actually have a workshop here about projection that you might find helpful: 
BPD BEHAVIORS: Projection .
I am glad that you and your sister are so close. Have you ever talked to her about your feelings of guilt about 'running away' when you were in your teens? I understand your feelings but do want to say that you were in your teens and just a child yourself. You were doing what you needed to do to protect yourself. One of my sisters was treated as the all-bad-child by our mother and also left home when she was just a teen. After she left things did become more difficult for me, but I still totally support her decision because she was treated very badly by our mother. In fact, when I was 10 I already had thoughts about not wanting to live in the same house as my family anymore, so I understand why my sister did what she did. She was my older sister, but she wasn't my mom and was still very young herself. Leaving was what she needed to do to protect her own well-being.
You say that you and your sister don't ever want to have children. Why is that? Have you always felt this way? Are you perhaps fearful that you would treat your children the same way your mother treated you? Or perhaps fearful that your mother would interfere in the lives of your children?
Take care