Jenny is the ninth model of the XJ android series, but when she discovers something downstairs in the basement, Jenny finally realizes she has the XJ-Sisters, which are prototypes of her. She is referred to as the eldest sister, despite being made as the ninth experiment. In regards to maturity and responsibility, Jenny represents the older sister in that sense.
The XJ-Sisters make their first appearances in the Season 1 episode, "Sibling Tsunami", and go on to appear in "Sister Sledgehammer", "Agent 00' Sheldon" and "Escape from Cluster Prime" (mostly cameo), and notably "Turncoats." (XJ-8 also appears briefly in "There's No Place Like Home School").
In "A Robot For All Seasons," Nora Wakeman was seen designing plans for an XJ-10 after Jenny was believed to have gone renegade due to falling under the control of Todd Sweeney. However, XJ-10 was never actually created.