Sis is Billy's aunt, Nergal's wife, and Nergal Jr.'s mother. In the episode "Skeletons in the Water Closet," she is revealed to be Harold's sister. Aunt Sis is a laid-back woman, who often looked bored until she fell for Nergal. Then, Aunt Sis began to smile a lot more.
Aunt Sis also usually has a voice that sounds like she has a stroke.
Aunt Sis fell in love with Nergal when a beehive fell in his pants, and he started to run around like crazy. Aunt Sis thought that he was dancing and instantly fell for him. Her first true love was actually a great dancer whom had stolen her heart. It was the greatest 4 and a half minutes of Aunt Sis's life, until he fell for another woman and dumped Sis, according to Gladys. This led her to become a spinster (or "splinter" as Billy so gleefully refers to her as).
Aunt Sis' appearance is somewhat different than Gladys'. Aunt Sis has an ginger hairstyle similar to Gladys', and a larger nose. She also wears glasses, and is a bit chubby. She also appears to be older than Harold.