mind their own business?

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Do I have to mind their own business or not?, One day my cousin went to me and told me something. This is all about her husband telling that the brother in law of his wife(my cousin) told him that this brother in law confessed to him, that he has other woman.He told him confidential things. He told him that he can leave his wife with other prettier woman. He told him that he can not leave his wife because they have a daughter. The husband of my cousin narrated all of this to my cousin. My cousin now is bothered if she is going to tell it to her sister that her husband has another woman. She is asking my opinion on what to do, if she will mind her sister own business or tell the truth? she is thinking that if she tell it, it may ruin the relationship of her husband and that brother in law. I advised her to tell the truth to her sister because blood is thicker than water. Her brother in law is fooling her sister. This should stop. Can you give me more advice. I am also confused.