Talk to a trusted adult who isn’t directly involved — a school counselor, a relative you trust, a therapist. They can help you think through consequences without forcing you to take immediate action.
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.
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.