There’s an old saying: “Let no good deed go unpunished.” It’s a cynical acknowledgement that sometimes a person doesn’t appreciate what another does for them, no matter how many times they have received the help. Your sister is a classic example of someone who feels entitled to take and take and take without being appreciative of the enormous gifts of time, help, and attention you have given her.
It doesn’t matter why it started. You are both adults now. Part of growing up is to put aside childish patterns and to assume adult roles with siblings and parents. I don’t know if she is simply selfish or has a personality disorder. I can’t know what goes on with her without more information.
But I do know that it is long past time for you to draw some clear boundaries about how she can treat you. You don’t owe her a thing. You don’t have to accept responsibility for her bad choices or to tolerate being put down.
You can’t change her. She sees no need to change. After all, you have continued to be helpful regardless of her disrespect.
You can change yourself. If you can’t find the inner strength to start saying a big “no” to her (and mean it), then you would benefit from getting some therapy to help you. A therapist can not only give you good advice about how to handle someone like your sister, she or he can also provide you with the support you need to change how you respond to her provocations and her pleas for sympathy. It would probably be helpful for your partner to go along to learn how to support you at home.
Please follow through and get the help you need. With better boundaries, you will feel better and it might even help your sister grow up.
I wish you well.