Hello pumpkin2020,
Welcome to the forum
I have a dBPD mother and an older sibling who has some sort of personality disorder. There was always tension and upset when I was growing up. My parents tended to reach out to me to take on a role like peacemaker, hero, parent to smooth things over. It didn’t resolve emotional upsets but placed an unfair burden on any one of us at any time. As a child, I was often put into a parent role to calm down my dysregulated mother. It was an overwhelming and painful role.
That’s called the Karpman Drama Triangle because two people who are upset reach out to a third person for balance. . It could be happening to you and your parents as they as reach out to you as a bridge to regulate your dysregulated sister.
No one can force your sister to act like she doesn’t have a personality disorder.
My sibling is similar to your sister where they can be charming and say nice things. It’s kept me off balance. I yearn for a loving family. I have to remind myself of their cruel behavior towards me and lack of remorse. What’s helped is not trying to help anyone to act reasonably.I don’t participate in any Karpman triangle when offered. I do low contact and it keeps me safe from the harm my family members caused me and can cause if I let my guard down.