When I first started having symptons I didn't believe there was really anything wrong with me either and because of this it was several years before I went to a professioal.  In my case it was the paranoia that kept me from going.  I was on the fence as to whether the things happening were real or not.    Recently, i came across a book called Schizophrenia for Dummies.  One of the things it said was that sz's usually have irrational fears or become paranoid about things to such a degree that no matter how much you try to show them logically that what they believe doesn't make sense or is irrational they will not believe you.  If I had known that this was a truth about schizophrenics way back in the beginning or even in the midlle I can't tell you how much that would have helped me.   Your sister's reason for thinking there is nothing wrong with her could be different from what mine was,but I recommend getting this book, reading it yourself (it's actually written for people who know someone who has it) and then showing her the pages that explain the syptoms and asking if she realizes any of them are true of her or you could ask her if any of those things are true of her without explaining why at first and if they are then you might want to show her the list of symptoms and the definitions/explanations in the book.  Sometimes seeing them all laid out and in writing can make it more real or believable.   In any case, I wish you and your sister the best.