Sister repeating a grade, hasn't told parents

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Alright, let me put some information down for the sake of context.

>middle class 3rd worlders, ie. equivalent poor 1st worlders
>sister is supposed to be doing her 6th and last year of secondary school this year
>mom and dad have been paying a massive sum of money since last year for her to go on her end-of-school trip with the rest of her classmates
>sister has a boyfriend, they've been together 3 years, and he's a good kid
>my sister just turned 18, is now a legal adult, and as such the school doesn't report anything to our parents anymore

I just got a message from her boyfriend. A friend came clean and told him that she's been hiding from both him and our family that she repeated and is going to have to do 5th grade again, not the final 6th grade as she should. Before you all ask, it's 100% confirmed, no mistake. He's tearing her a new asshole over the phone right now, because she doesn't want to come clean. Thing is, her boyfriend knows my parents aren't going to take his word over hers (for various reasons, and from my point of view he's completely right), and he really, really doesn't want to lose my sister over this.

I can imagine my parent's reaction after finding out, especially since they've been working ridiculous overtime to pay for that 10-day-long graduation trip that comes out to about a month's wage. It's absolutely terrible. I imagine the reaction of them finding out after the trip, knowing that she deceived them for multiple months, to be a billion times worse. I want her to not have to suffer so terribly for her stupidity.

My problem lies here. Normally, I would do what's best for her even though she'll hate me for it, and spill the beans. But my sister has some triple-S-grade dirt on me, and I'm worried that in her spite, she'll give my parents the dirt on me. Please don't ask further about that dirt.

What can I do?