Could you invite your parents to lunch? Or a movie? Are they well enough to go on an outing with you? You can try to do something with them outside of your sister's house. If they aren't well enough for that, I would probably just suck it up and deal with your sister in order to spend time with your parents. They are old and you could never see them again, and your parents could pass thinking you don't want to see them. Putting up with some **** from your sister might be worth it just to see them.