Sister Cat is a young kitten. She wears a pink tie and a flat pink and white hat with a flower on top. She gets a bit carried away imagining a Glunk that cost her family a fortune with a long-distance phone call to his mother. After Young Cat in the Hat helps her unthunk him, she learns a valuable lesson. She is known to be the daughter kitten of The Cat in the Hat due to resemblance. Young Cat is her older brother and her great great-great-great grandfather is King Looie Katz.
She appears in the book I Can Lick 30 Tigers Today! and Other Stories.