Mahouka Koukou no Rettousei: Yotsuba Keishou-hen

Miyuki Shiba is summoned to the Yotsuba family estate for the New Year's reception that will choose the next head of one of the country's most dangerous magical clans. The brother she has spent every series volume devoted to — Tatsuya — is, on paper, ineligible to attend. The current head opens the proceedings by claiming Miyuki and Tatsuya are not biological siblings. The film is the long evening that follows, the political marriage Miyuki is being asked to accept as a condition of the succession, and the answer Tatsuya finally gives when asked what their relationship has actually been.