Mahouka Koukou no Rettousei - Yotsuba Keishou-hen

In December, Miyuki receives an invitation for the Yotsuba's new year's family gathering. However, unlike previous years, they specially added "Everyone must attend" on the invitation. Miyuki understands that "the time has come" — it is time to decide the next head for the Yotsuba Clan.
— If she is elected, her brother will be free.
— If she becomes the next head, she will be forced to marry someone else.
Miyuki's heart is a mess.
The truth of the Brother and Sister relationship is finally unveiled!? Yotsuba Keishou-hen begins!

Adaptation of the original novel's volume 16.