My mother-in-law wants to bring her husband's brother's 3-year-old daughter to my home so that her children can go to school here. What should I do?

It is understandable that you don't like it. Because selfishness often triumphs over compassion. Turn people into demons. If you don't like the little girl coming, never let her come. It's better than the little girl coming and you being unkind to her. My aunt lived in a relative's house for a while when she was 4 years old. The host loves her very much, but the hostess abuses her. Throw her clothes in the toilet, slap her when you come in, and slap her when you go out. I left her a lot of work ... then my grandmother found out and took my aunt away. Unexpectedly, twenty years later, my aunt married a senior official in a first-tier city. Half the relatives in the family are exposed to the light of menstruation. Including me. She paid for my college tuition.