Let her know that this is her own brother and the closest person in the world in the future. In many families, if these sisters quarrel with their younger brothers, many parents will subconsciously protect their younger brothers and blame them, but we should know that they are actually children and don't understand many things. If you criticize and scold them directly, I think it will often be worse.
We all know that it is normal for children to quarrel, but the key lies in how parents solve it. If my sister often bullies my brother and steals from him, then we can find an opportunity to find two children together and ask her if her sister robbed her brother, why she robbed him, and whether her brother didn't listen to her. At the same time, we should tell them that they should help each other and they should not quarrel. Sister should not bully brother, brother should listen to her.
Now my sister wants to protect my brother. When they grow up, their younger brother will protect their older sister. Then, we should give them proper time to get along and let them handle some contradictions by themselves. Parents should participate as little as possible. I think they will understand in the slow running-in process. Also, you can't criticize older children just because you are old. In fact, the right or wrong of children has nothing to do with age. You can't let them have their own big children from an early age, but let them be small.