My brother-in-law often sends text messages to harass me.

Time and time again, two is not three. It's important to protect yourself. Give my sister a general idea first. If my sister believes that you are wary and act, my brother-in-law will not go too far and don't let everyone know. If my sister doesn't believe that you and my brother-in-law are still doing this, I suggest you poke it out directly in front of all your family. After all, you warned your brother-in-law and told your sister before, but neither of them did anything, so I really can't blame you. The happiness of their husband and wife is very important, and so is your own happiness.