I just found out my dad is cheating on my mom. Who do I talk to?

There's much more to this story than just the title. However, I see that I only have 5000 characters, so I'll do my best to summarize. Today, on my way home, I saw that my dad had a missed call on his phone. I opened the screen to see who it was, and right below, I saw that he had an unread text from an unsaved contact. I recognized the number, so I entered it into my own phone and found that it matched the cell phone of my aunt (mother's brother's common law wife). It seemed a little over-affectionate, so I opened his inbox and outbox, purely out of curiosity because I'm only human. Much to my horror, I found a whole slew of messages, sent and received, between my dad & my aunt. Some of them were really explicit, and one even said something hurtful about my family. There is no way that I could ever confront my dad about this - I can hardly look at him. The idea of talking to my mom scares me. She stopped working once she had her first kid (I have two siblings in their 20s), so my dad brings home the bacon. If she wanted a divorce, it would hurt her not only emotionally, but financially, for sure. I don't have the heart to talk about this with my siblings, and also am afraid of how they would handle the situation. What's even worse is that I know this must have been going on for a long time. See, my dad's father died a few years ago. Since then, he's been taking a lot of weekend trips to "see his mom", but the texts I read made it clear that they have been meeting in the city where my grandma lives. It sickens me that he's been using my grandma, who is pushing 80 and has Parkinson's, as an alibi. Since this affair involves not only my dad but my AUNT, I can't just talk to anyone in my family. This is very heavy, but delicate at the same time. At my age (17), the only person I trust enough to talk to about this is my best friend, who is out of the province for two weeks. Since I don't have my full driver's licence, I can't seek outside help without my parents finding out either. Help, please.