"Congratulations! They're twins."
During an adventure in the dream realm, A suppressed memory/flashback of Rose's infancy revealed not only what her parents had looked like, but that she also has an unnamed identical twin; slightly older, having been born first.
She also learned the truth of how the Huntsclan stole her from her family as a newborn due to the Huntsclan symbol, a dragon-shaped birthmark on all naturally born dragon slayers, on her left wrist and hand, which is the only identifying feature to tell her apart from her other half.
She is only seen in a flashback/dream in the episode "Dreamscape" as a newborn on the left, being held by her mother. For unknown reasons, she remains unseen in "Homecoming" and the series finale, even though Rose's history was rewritten by Jake's irreversible wish; as neither Rose nor their parents seemed to have any knowledge of her existence that matched Rose's.
Some fans thought of giving another flower name like Rose; either Lily, Daisy or even Violet.
She and Rose are the second and last set of twins after the Oracle Twins.