Sherri and Terri's sister is the sister of the twins Sherri and Terri. She was mentioned by J. Loren Pyror during his meeting with Bart; during this, he stated that Sherri, Terri, and the unnamed sister were conjoined triplets and the third triplet is out seeking revenge after being separated from her sisters. However, this may just be a throwaway joke by the writers of the episode, and may be considered non-canon until the supposed third sister officially appears.