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:
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 caught my stepmom cheating
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. I’m really sorry you’re in that situation —