What kind of psychology is it that elder sister always belittles younger brother?

My sister always belittles my brother. I think there are many kinds of psychology. First, he may be jealous of his younger brother, and then he thinks his younger brother is better than him, so he is jealous of his younger brother. Another possibility is that he is trying to motivate his younger brother to study better and make him better. The last possibility is that he may have two children at home. He wants to be first and always above dd, so he belittles him.