Dear Prudence is Slate’s advice column. Submit questions here .

Dear Prudence,

My sister “Candace” spent years cheating on her husband. I knew about it, but never said anything because I felt it wasn’t my place to interfere in her marriage. My 15-year-old niece , “Tamara” recently found out the truth when Candace left her phone on the kitchen counter and received a lurid text that Tamara saw. She then confronted her mother about it. She was so furious that she moved out to live with her dad. The problem is that my sister threw me under the bus by telling Tamara that I knew the entire time, but kept my mouth shut. Now my niece hates me because she thinks I should have told her dad about it. Was my approach of keeping silent about my sister’s infidelity the correct one to have tak

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