And your parents allowed her to get away with thinking this way instead of using her behaviors as a child as a teaching her opportunity. We all are a product of our environment as we perceive it, good or bad unless we are shown/taught that a different option exists. It is sad when it ends up like your sister because it is who she has become.....but what she is, is the reality that has existed all along through the development of that personality never being offered an alternative direction from the beginning.
Some of us learn alternative directions by not liking what we experience, from external sources (like our education), or best, functional parents who know how to handle challenges within their family.
We all experience this at one level or another