Is BPD really an excuse to be abusive to your kids? My sister says so

My mother has BPD and severly abused me as a child. I felt like a literal punching bag. Constant spankings and face slapping for every little thing. It was like impossible to not get hit no matter how well I behaved. She always thought I was evil and scheming and would give me extreamely humilating punishments as a child. She would also say mean and hurtful things to me. I was also severly medically neglected and abused in horrible ways that I wont dive into but it left me with chronic pain as an adult. I was the family scapegoat while my sister was the golden child. My sister was the golden child and my mother and father rarely put their hands on her. She now acts very smug and makes excuses for my mother and says that “she was not that bad” and that “she could of been worse”. However I feel like she only says this because she was treated fairly well and to this day my mom still treats her like a princess. She also says that my mom could not control her actions because she has borderline personality disorder. She also uses the excuse that she had a bad childhood. and that somehow excuses the abuse that she inflicted on me? My sister smugly defends my mother but has no empathy for what I have been through.