Sorry, without know more I can't tell you for sure. I do however, suffer from both myself amoung other things, such as explosive personality disorder.
So the only thing I can provide for you is my prayers as you will need strength to see you through & lots of guidance of how to help your mother heal.
I have both BPD and PTSD. Your mom may too. There should be a checklist of symptoms on the DBT site by Marsha Linehan. It's definitely in her workbook. If your mom answers yes to most of the symptoms, then she probably has it. It sounds to me like she does. Good luck in finding out.
Its really hard to say about BPD vs. PTSD however most BPD have major unresolved trauma. I know this is true for my mother and she is borderline. She is difficult and makes my life difficult. I have asked her about dealing with her trauma and she states that it does not effect who she is today.
At least your mother is seeing a psychiatrist and getting help for PTSD is a great start regardless of the diagnosis.