My sister-in-law does nothing, which makes me very angry. What should I do?

I haven't married him yet, but I usually go to his house for dinner more often. Usually, my mother-in-law does all the shopping, cooking and cleaning. Just today, before noon, his sister didn't come home for dinner. I saw his mother buy breakfast, food and cooking. It's hard. So I help wash the dishes. In the evening, five of us (me, him, his father, his mother and his sister) have dinner at home together. Because it's almost 8 o'clock after dinner, I said I would go to the hospital to see my boyfriend's aunt without washing dishes. His sister said she couldn't go because her sister didn't like this relative very much, so it was just me, him, his father and his mother. Because the hospital had visiting hours, the dishes were not washed. It takes about 3 hours from going out to going home. His sister has been staying at home, not going out, staying at home and not washing dishes. When my boyfriend came home and saw that the dishes were not washed, he put away the chopsticks. His mother washed the dishes, but she didn't do it. The point is that no one told her that she didn't wash the dishes and didn't ask her to do them. She usually does nothing at home. I asked him why you didn't say anything about your sister. He said she was stingy, and a few words made her unhappy.

His family usually indulges his sister, and so does my classmate's mother-in-law. My classmate's sister-in-law gave birth to a second child and took it home every day. My classmate's mother-in-law only helps her daughter, not my classmate. I'm really afraid that my grades will be the same as my classmates. I also told my boyfriend about it. He said everyone is different. Just because his sister is like this doesn't mean that my sister is like this. I said, but your mother dotes on her daughter as much as her mother, and probably so. I also thought that it might be better when his sister got married. But his sister is older than me, has no boyfriend and doesn't want to get married.