What I did in the summer vacation was to guide him.
You should respect her, don't treat her like a child, pay attention to your tone when you talk to her, and don't act like an adult, she will resent it.
You can ask her what you are thinking and tell me. But she told you, and you have to promise not to tell your parents, so you can be trusted by her.
I hope she is filial to her grandmother, so be kind to her grandmother, especially in attitude, and tell her not to take her kindness to you for granted.
When eating, tell her not to think about giving herself delicious food and giving others bad food. No one likes to eat bad food. We must form the habit of sharing good things with everyone.
Anyway, educating children has a long way to go, and it is not temporary. You need to refuel. Anyway, my brother and I are the best in the family. He has a bad temper, but he can listen to me.