Why does elder sister-in-law hate younger siblings so much?

To put it simply, the elder sister-in-law generally loves her younger brother, but is not good for her brother-in-law. This is a traditional family problem in China, just like mother-in-law and daughter-in-law are hard to contact. This is a family problem for thousands of years.

My sister-in-law feels that she has the advantage of being a girl at home, so her daughter-in-law has to listen to her. Such people are generally blind.