Should I have exposed them privately?

AITA for exposing my fiance during our own engagement party after I found out what he really did with my sister. My name is Rachel and I am thirty one years old. I was engaged to a man named Derek for almost two years. We planned an engagement party at my parents house with family and close friends. Everything seemed perfect until three days before the party. I borrowed his laptop to print some invitations because mine was broken. While searching for the file I found a folder labeled vacation photos. I opened it out of curiosity. Inside were messages between Derek and my younger sister Amanda. The messages were not innocent. They spanned almost eight months. They talked about meeting secretly during family holidays. They talked about a weekend trip to the coast that I thought was Derek visiting his cousin. My hands were shaking as I read every single message. I felt sick reading how they planned to tell me after the wedding once things were more stable financially. I did not confront him right away. I decided to wait. I wanted proof that would be impossible to deny in front of everyone who mattered. I copied every message and photo onto a flash drive. I smiled through dinner that night like nothing was wrong. Derek even joked about how excited he was for the party. Amanda sat across from me laughing at his jokes like she had not betrayed me for months. The night of the party arrived. The backyard was decorated with fairy lights and white flowers. Nearly sixty guests came including our parents grandparents and childhood friends. Derek stood beside me holding my hand pretending to be the perfect partner. When it was time for speeches he grabbed the microphone first. He talked about loyalty and trust and how grateful he was to have found his soulmate. I almost laughed out loud at how easily he lied. When he finished I asked for the microphone. Everyone assumed I would talk about our love story. Instead I connected my phone to the speaker system already loaded with screenshots displayed on the projector screen behind us that my cousin had set up earlier for a slideshow. I clicked play. Message after message appeared showing dates times and explicit details of their affair. Gasps spread through the crowd almost instantly. My mother covered her mouth in shock. My father stood up so fast his chair fell backward. Amanda turned pale and tried to grab my phone but I stepped away holding it high. Derek started stuttering trying to explain that it was not what it looked like. I looked him straight in the eyes and told everyone calmly that our engagement was over effective immediately. I told Amanda that she was no longer welcome in my life. The backyard fell completely silent except for someone crying near the back. Derek tried to grab my arm begging me to talk privately but I pulled away. My father stepped forward and told him to leave the property immediately. Amanda ran inside sobbing while our mother chased after her confused and heartbroken. Guests started leaving awkwardly not knowing what to say. Within an hour the party that was supposed to celebrate my future instead became the night my entire family fractured. The following weeks were brutal. My mother tried convincing me to forgive Amanda calling it a mistake of passion. My father barely spoke to Derek family after the incident spread through our small town. Amanda tried texting me apologies almost daily but I blocked every number she used. Derek moved out of state within a month claiming he needed a fresh start. Some relatives blamed me for humiliating everyone publicly instead of handling it privately. Others said I did exactly what needed to be done. My best friend told me I was brave for standing up for myself instead of pretending everything was fine. Six months later Amanda reached out through my cousin asking if we could ever repair our relationship. I still have not answered her. Every time I think about forgiving her I remember reading those messages alone in my room questioning everything I believed about my own family. Now I constantly wonder if exposing them publicly was the right choice or if I destroyed relationships that could have healed quietly over time. Was I wrong for turning my own engagement party into a public reckoning instead of confronting them privately first.