I Admit My Sibling's Lies

For years, I've caught my younger sister lying to our parents about her grades. She'd get perfect scores on tests and claim Cs, all while keeping a straight face. Every time I see her do it, I feel a weird mix of anger at the lie and admiration for her acting skills. I know it's wrong not to say anything, but somehow, part of me feels she deserves a chance to be this dishonest without consequences.