Sister and Fiancé Betrayed Me at the Dock

AITA for exposing my sisters affair with my fiance at our own engagement party. I need to know if I went too far or if she deserved every second of it. My name does not matter here. What matters is that three years ago I introduced my sister Rachel to my boyfriend Derek. They got along great. I thought that was a good thing. I was wrong. Fast forward to last month. Derek proposed at a lake house we rented for the weekend. I said yes. I cried. My family cheered. Rachel hugged me tighter than anyone. She whispered congratulations in my ear and I felt nothing but love for her. That same night I could not sleep. I went downstairs for water and heard voices near the dock. It was Derek and Rachel. They were laughing quietly. Then I heard something that stopped my heart. Rachel said we cannot keep doing this after the wedding. Derek said we will figure it out later, just relax and enjoy tonight. I stood frozen in the dark kitchen. My hands were shaking so hard I dropped my glass. It shattered on the tile floor. They both turned toward the house. I ran back upstairs before they saw me. I spent the rest of the night staring at the ceiling, replaying every memory of the last three years. Every late night Derek said he was working. Every weekend Rachel claimed she was visiting friends. It all made sickening sense now. The next morning I acted normal. I smiled through breakfast. I let Derek kiss my forehead. I hugged Rachel and told her how much her support meant to me. Inside I was planning something. Two weeks later we threw our engagement party. Family, friends, coworkers, everyone was there. I waited until the toast. Derek stood up with a glass of champagne, talking about how lucky he was to find true love. I stood up right after him. I smiled and said I have an announcement too. I said Derek and I have decided to share something honest with everyone before we get married. I looked directly at Rachel. I said Rachel, why do not you tell everyone about the late night talks by the dock. The room went silent. Rachel turned pale. Derek dropped his glass. I continued calmly. I said I heard everything. The secret meetings. The lies about work and friends. The whispered conversations about after the wedding. My mother gasped. My father stood up furious. Rachel started crying, saying it was not what I thought. Derek tried to grab my arm and I stepped back. I said do not touch me ever again. The party turned into chaos within seconds. Some relatives left immediately. Others stayed to comfort me. Rachel ran outside sobbing. Derek kept trying to explain himself in front of everyone. I did not care anymore. I called off the engagement that same night. I blocked both of them everywhere. My mother tried to convince me that Rachel made a mistake and deserves forgiveness. My father agreed with me completely and stopped speaking to Rachel altogether. Now my family is split in half. Some think I should have handled it privately instead of humiliating everyone at the party. Others think Rachel and Derek deserved public exposure for betraying me for years. I have not spoken to Rachel since that night. Derek has tried calling me from different numbers, but I refuse to answer. I keep asking myself if I destroyed my family over one dramatic decision, or if I finally gave them the truth they all deserved to hear. So tell me honestly, did I go too far by exposing them in front of everyone, or did they earn every bit of that humiliation.