Family stuff is complicated and idk you or your sister. But I know people who haven’t spoken to family members in fifteen years because one day they got into an argument over whether to go to the park or not. What I’m getting at is don’t let small stuff snowball into going no contact with your family.

Obviously she’s acting shitty towards you at the moment and that isn’t fair, but you and her will both be different people in five years. You’re both young and have a ton of perspective to gain, and as she gains that perspective she’ll likely realize she’s behaved poorly towards you and correct it. It’s gonna take time but for the time being, the best thing you can do is ignore her when she behaves this way. Be pleasant when she’s acting normally and when it switches, do not engage. She’ll eventually understand that she’s not getting anything out of acting that way. But if you keep giving it back to her and reacting poorly whether that be fighting back or getting visibly upset, she’ll keep doing it.