Yeah. You fucked up. You are upset at her for having lied about her grades (those are still her grades not yours) but you went right ahead and felt justified in breaking into her account.

So are you now going to lie that you didn’t do it? Are you going to spend the rest of your life holding the secret? Or are you going to let it out and cause a tremendous amount (I’m assuming here) of drama and trouble and angst for everyone?

What you need to do is mind your own business. Your sister has her own path and her own life to lead and it is not up to you to make choices for her.