Originally, when trying to deceive Basara and his father, Mio displayed a personality very typical to that of teenage girl. While at home she acted like a normal sister towards Basara, waking him up in the mornings and referring to him as "big brother."
Read more at http://myanimelist.net/character/92789/Mio_Naruse#7Ajx8WBe0bIJrFkq.99