My father-in-law favors my brother's sister-in-law.

I can understand that you are all married daughters-in-law, and the different treatment is really a bit sour, but you should also look at what is going on. Are you not familiar with your family and have not established deep feelings when you first enter the door? Or are you not as good to your parents-in-law as your eldest brother's, and not as diligent and considerate as others? In fact, old people are just like children. You are good to him, and he is easy to be good to you, as long as he is not biased. The eldest son doesn't like the younger son's family. Parents are generally treated equally. As for the child who is too poor and loves the rich, most of his family members are bad. Maybe your mother-in-law likes your brother and sister-in-law very much and thinks they are good to themselves. Then you should be nice to them. They can see your heart and slowly improve your relationship.