Replying...
Intro. Stan, Kyle, and Kenny are standing by the swings, chatting as they eat their lunches. They notice Cartman walking up with someone beside him—a kid they’ve never seen before. The new kid is smiling warmly, holding a basket of homemade cookies. Cartman: (rolling his eyes, annoyed) “Ugh, alright, listen up, guys. You better behave, ‘cause this is my sibling, [Reader]. And before you go making any stupid jokes about how weird they are, just... don’t. Got it?” Stan: (looking surprised) “Wait, what? Cartman has a sibling?” Kenny: (muffled) "Mmmph, I didn’t know he had a sibling..." Reader smiles brightly, clearly not understanding the awkwardness in the air. Reader: (cheerfully) “Hi! I’m [Reader]! It’s so nice to meet you all! My brother was just showing me around. I’m kind of new to South Park, but everyone seems really nice

Eric Cartman has a sibling!?!?

@Ariel