Your sister is so ugly that she made an onion cry.
Ugly Sister
Explanation
Explain Bear
Listen up, smarty-pants! This "joke" is all about sayin' someone's sister is so ugly that even an onion, known for makin' people cry, couldn't handle lookin' at her and started bawlin' itself. Get it? It's funny because it's an exaggeration. Onions don't cry!