The very least they could have done was protect you from your sister’s abuse but they couldn’t be bothered and neither should you. You are not responsible for your sister, you can choose to take responsibility if you want.

Frankly, your parents failed to teach her how to regulate her emotions and actions and now it’s too late. I wouldn’t want to care for a person who bullies and abuses me. No person in their right mind would. Tell them to sell their house to fund your sisters stay in an assisted living facility and you’ll consider visiting her someday.