Brother-in-law provides food for customers.

Save the text message he sent you first and threaten him. If this continues, forward all the messages he sent to you to your sister. If he doesn't back down, just tell your sister. If your sister can't control him, tell your parents about it. He thinks you dare not make a mountain out of a molehill, so you are a hooligan. We must resolutely express our determination and show that we are not afraid of tearing our faces.

Put his mobile phone number on the blacklist.

This rotten man can't give your sister a long and stable life, and sooner or later, a mistress will come in. Stay away from him if you can.