In part two of "Hollywood Jem" Sean "the British teen idol" woke up to Kimber in his room. She told him
Kimber: "Now you better get better quick. You're holding up my sister's movie."
His response should have been "Wait. I thought Jerrica was your only sister. How is Jem your sister?"
Harrison_Ford_ROCKS!!
shareThat happens several times throughout the show. In Season 1, Jem refers to Kimber as her sister in Video's presence, for instance.
It can be justified by saying that people like Video and Sean assumed that the Holograms were so close to each other, they regarded one another as "sisters". After all, Shana and Aja were Jerrica and Kimber's foster sisters, so it's not a stretch to imagine that people viewed Jem as a surrogate "sister" to Jerrica and Kimber also.