"Congratulations! They're twins."
During an adventure in the dream realm[1], A suppressed memory/flashback of Rose's infancy revealed not only what her parents had looked like, but that she also has an older twin sister, whose name is unknown.
She is only seen in a flashback/dream in the episode "Dreamscape" as a newborn on the right, being held by her mother. For unknown reasons, she does not appear with Rose and their parents after the defeat and banishing of the Dark Dragon. It is unclear what truly happened to her.
She and Rose are the second and last set of twins after the Oracle Twins.