Hi Cheeemilee

A family-member with BPD can really impact the entire family system. I am sorry you are struggling so right now with the situation with your sister. She has been diagnosed with BPD, is she currently being treated for it? Is she perhaps getting targeted therapy for her BPD?
We also have a middle sister, age 21 who my sister adores and doesn't have any issues with, which hurts even more.
It hurts so much that she hate me like this, and maybe it is the mental illness talking but she's so young, I'm scared we'll never be close again and she will always hate me.
It sounds like your sister is engaging in the BPD behavior known as 'splitting', are you familiar with this term?
Splitting refers to a primitive defense mechanism characterized by a polarization of good feelings and bad feelings, of love and hate, of attachment and rejection.
Splitting is a powerful unconscious force that manifests to protect against anxiety. Rather than providing real protection, splitting leads to destructive behavior and turmoil, and the often confused reactions by those who try to help.
... .
Individuals suffering from borderline personality disorders live in an immature psychological world, fueled by certain constitutional vulnerabilities, where they attempt to shield themselves from conflict and anxiety by splitting the world into all good and all bad. Although this produces an sense of psychological safety, in fact, it renders relationships fragile and chaotic and drives away the very people who are so badly needed to provide stability in the borderlines life.
We have a workshop about this subject that you might find interesting:
BPD BEHAVIORS: SplittingWe describe certain tools here that might help you in your communications with your sister. There is never a guarantee that it will work of course, but the tools can increase the likelihood of having calmer interactions with your sister:
Communication Skills - ValidationCOMMUNICATION: S.E.T. --> Support, Empathy, Truth