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 and began to smile a lot more.
She was the love interest and later wife of Nergal in Love is Evol Spelled Backwards. She was voiced by Grey DeLisle.
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).