What do you do when you have a brother in law who is talking shit about you behind your back to your wife?

I'm not going into details, but he said some things about me, this man who I have spoken to in years, who I never speak about, started talking shit about me behind my back to my wife, upsetting her, making her cry. And so on.
What do you do when you have a brother in law who is talking shit about you behind your back to your wife?
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