I would not tell, but I would hint to your parents to check it out for themselves. Don't feel bad. The truth will come out eventually. Or get a teacher/administrator.

I'd say at this point just continue doing your best, your marks will gradually improve. Your sister dishonesty will catch up to her eventually

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.

Tell your sister what you did and apologize to her - it was wrong to hack her account. Then take it no further, and make sure she realizes that you won't tell anyone. She must deal with it as best she can, any further interference from you would just make things worse.

Mutually Assured Destruction, motherfucker! The next time you lie, or even if someone thinks you do, you have justified them hacking into your shit

This! Just be like “oh mum, dad, I found out you can check our reports online, I’ll show you how to do it on mine” and then even drop a subtle “I wonder if sister knew she could check this way, you should ask her” or similar

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Don't feel bad about your grades in America. The schools here are pretty much just state subsidized day care.

I would ask that both of yo be placed in a private school where there would be greater interaction with staff where the feedback loop to your parents could be augmented.

You wouldn't have to do anything more.

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She’s lying for the same reason that you want to expose her lying, to try and escape the scrutiny of your parents. Your parents are really fucking you guys up if all they do is insult you for not achieving high marks, the best thing you can do is tell her that you know and actually support each other instead of fighting for your parents approval.

...as long as he doesn't tell anyone (other than reddit).

This isn't your fight. Leave it be. Live YOUR life. You're too old to be narcing on your sister about grades.

As others have mentioned, you're offended by her dishonesty...but you violated her privacy? Nah. Move along.

This is the Neutral Good response, I like your take

Reminds me of the girl who lied about getting into university and was actually living with her drug dealer boyfriend for years and when the parents found out she staged a burglary and tried to have her parents murdered but only succeeded to kill her mom. It was absolutely nuts. https://allthatsinteresting.com/jennifer-pan

I got good grades and that probably set me up for a good start for my career but it certainly hasn’t mattered since I started my first professional job out of college.

Definitely the soft skills and working well with people is more valuable in most professional environments unless you are the best at some high value niche specialty then those soft skills can slide