You can get on well with him first, and then tell him some of your little secrets (irrelevant). This secret is very important to you, and you should tell him not to tell your sister, and pretend to be annoying. The above is the first step to gain his trust.

Step 2: You can test him a little. After venting your dissatisfaction with your sister, let her ask weakly: Do you think she is annoying, too? If he doesn't answer, maybe he found out, then you can say as a family member: tolerate my sister more in the future.

Step 3: If he finds out that you are testing him, it means that he is smart. Then we have to adopt another plan: you can create some embarrassing scenes for your sister and see how he can solve it. Or pay attention to whether he remembers your sister's birthday and knows what your sister doesn't like.