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Sei's Onee-sama is not given a name, but appears in flashbacks. She apparently chose Sei because of she liked the latter's beautiful face (she said she chose Sei for her face so as to not give the latter any pressure or expectations). She does not pressure Sei to find a petite sœur, nor coerce her into making Shiori into one; rather, she tells Sei to choose her own path without any regrets. In the third novel, Sei describes her as someone who is noble enough to be worthy of the Gigantea title; she really does, care for Sei, as shown after Shiori leaves. Sei began to depend on her a lot after this too. She graduated from Lillian wishing Sei to find a good petite sœur.
Source: Wikipedia