No Kitty Cat, that is not justifiable at all.
Sometimes, family members tend to employ narcissitic personalities, and view others as extension of themselves. You exist only to serve them, to make them feel better. You have to deal with it else it will only fuel her attitude towards you. You need to put a stand to it. Show her that you are not going to stand for her ill treatment. Do not help her as much as you do. Whenever she puts you down, tell her straight up that you dislike her attitude and walk away. Be mad at her. Let her apologise. Do not listen to her problems as much, and generally shut her off until she gets the hint and realises the error in her ways!