She is mentioned by her mother, Betsy Donovan, who was trapped in the toilet due to Clyde leaving the seat up. Betsy tells Clyde to always lower the toilet seat for his sister's sake. During Betsy's funeral, where several relatives of Clyde are shown, a teenage girl can be seen sitting on the right side of Clyde and his father. It is possible that this is Clyde's Sister. In a deleted scene from the episode, a girl is seen singing at Betsy's funeral; it is also possible that this could be Clyde's sister.
Clyde's first possible sister has curly brown hair, brown eyebrows, and red lipstick. She was spotted wearing a long-sleeved black shirt and gray skirt at the funeral. His second possible sister has short brown hair and brown eyebrows. She wears a pink shirt, a black skirt, black shoes, and glasses.