You know sometimes you are just attracted to someone and you can't help it? And you end up being really goofy around them, and can't be relaxed? I get this sometimes, and I have no control over it! It doesn't mean I want to have sex with that person!! It just means there is an attraction- an appreciation, that is chemical, and you can't help it. It has nothing to do with real life or love. I wouldn't worry about it. So long as your sister doesn't fancy him, nothing will ever happen!