I want to see how much money he gave his sister. If the denomination is small, just open your eyes and close them. If the denomination is a little large, tell him gently. Although he may be afraid that you don't want to be unhappy, he should at least have some respect. After all, a family is run by two people, right?

I want to ask you what kind of attitude you would have if he told you. If he will accept his approach, then tell him that he will discuss everything with me in the future. You can ask why you gave her money. If you think it's too much, discuss whether it can be less, provided that you can't rush it, otherwise it will feel different. If he tells you that you can't accept his behavior, that's another matter.