Shiba Miya is the mother Tatsuya and Miyuki. She is a member of the Yotsuba Family and the twin sister of the current head, Yotsuba Maya. However, prior to the start of the light novel series, Miya died due to poor health. Her first appearance is during the Reminiscence Chapter that happens in 2092, three years before the current timeline. (Source: Mahouka Wikia)