Friendship & Infidelity

My best friend had a one night stand with my husband's brother while he was taking a break from his then girlfriend 4 year's ago. My husband and I did not know about the one night stand between my best friend and his brother until last summer when his brother was getting ready to be married and expecting a baby with the on-again off-again girlfriend. I did not tell my sister in law to be that I knew about the one night stand because I assumed they had worked things out if they were getting married and having a baby- besides it happened 4 years ago! Fastword one year later and 2 kids later she comfronted me about the one night stand. She said her marriage was failing and she needed to know the truth so I was honest with her. I didn't know when it happened and I didn't know about it while we were friends. I feel like it wasn't my place to say when I found out last summer. Now she's hurt by me too and won't respond to my texts or answer my phone calls! Was I truly not being a good friend and sister in law by not telling her last year about something that happened 4 years ago while they were taking a break?! Need advice.