My sister is an idiot, how do I make her understand?

There will always be people like that, I know a few..and there isnt much you can do to make them see how much they are hurting you. Sometimes though, they are very attention oriented and thrive on the way you react and the problems they create. So a good way of going about it is to boot her out of your life as much as you can and just ignore her. She will get the picture..eventually. hopefully..