I’m really sorry you’re in that situation — it’s a heavy and confusing thing to witness, especially when it involves someone close to you. While I can’t know all the details, here’s a proper, grounded piece of advice on what you might consider doing next:
Families can fracture. You might be blamed by her, or even by your dad temporarily if he shoots the messenger. None of that means you did the wrong thing — it just means the situation was already broken. i caught my stepmom cheating
You could say calmly: “I saw something I shouldn’t have. You have 24 hours to tell Dad, or I will.” This puts the responsibility on her, but only do this if you’re prepared for her to react badly. I’m really sorry you’re in that situation —
If your dad is a good parent to you, he deserves to know — but how and when matters. If telling him could put you at risk (e.g., he’s volatile, or she might turn things on you), prioritize your safety first. None of that means you did the wrong
If you want to talk through the specific details more privately or need resources, let me know — I’ll help as best I can.
Are you 100% sure it was cheating? If yes, write down what you saw (time, place, actions, words). Not for a courtroom — just so you don’t doubt your memory later.