Behind The Scenes
In a scene deleted from the final version of "Cartman Gets An Anal Probe", Cartman wonders if his mother's worried about him. She notices something's missing while serving her husband and daughter, then realizes... it's the butter.
When South Park was picked up by Comedy Central, Trey and Matt decided Cartman didn't need a larger family, and the identity of his parenthood remained clouded in mystery until season 14. His only canon sibling is his evil half-brother.