My boyfriend's big sister is mean to me.?

That sucks!!! When I'm mean to my brother's girlfriend it's because she treats him like poop so maybe you should try treating him better. If that's not it then block her from your myspace and take control of it that way. Also if he likes you he should ask her to stop and if he doesn't and you guys break up in the future you will find sooooo much better. Kill her with kindness always works too.