It's unfortunate that no one apparently tried to help your sister deal with this problem when she was growing up. As time passes, we often become more and more set in our ways -- and it becomes harder and harder for us to change. But God still can change her -- and I hope you'll be praying for this to happen.
On the surface, your sister may appear happy -- but in my experience, people who are negative and critical toward others usually aren't very happy at all. One reason is because no one wants to be their friend, for fear they too will be criticized and cut down. A highly critical person also has a hard time loving others and accepting them just as they are. The Bible says that true love "keeps no record of wrongs" (1 Corinthians 13:5).
Why is your sister this way? I don't know all the reasons, but I do know that people like this often tear others down in order to build themselves up. In other words, it's their attempt to convince themselves (and others) that they must be better than anyone else. But this is doomed to failure, and only shows how insecure they are.
Pray for your sister, that she will discover the greatest truth anyone can ever discover -- that God loves her, and wants to be her friend. Then ask God to give you an opportunity to confront her -- lovingly but clearly -- about her attitude toward others. In addition, let her know you still love her in spite of her negative attitude -- and so does Jesus Christ.