Kobeni's now engaged to a total stranger. Her fiancé and his sister move in, things get even weirder.

Just an ordinary teenager, Kobeni Yonomori receives quite the surprise on her 16th birthday-a fiancé and a sister-in-law she never even knew she had. As a result of an arrangement that her late grandfather made, Hakuya Mitsumine and his younger sister Mashiro have moved from their countryside home to the Yonomori household in order to deepen their relationship with their new family members. Mikakunin de Shinkoukei follows Kobeni's "love life" with Hakuya as she tries her best to adjust to the abrupt changes forced upon her. However, as some extraordinary secrets regarding the siblings come to light, Kobeni will find her life changed forever.—MAL Rewrite

Not every girl gets what she wants on her 16th birthday, but Kobeni's gift is something she never saw coming: an arranged marriage. Thanks to her late grandfather, Kobeni's been engaged to a total stranger since childhood, and even though she's supposedly met him she doesn't remember a single thing about him. Even weirder is that both her Mom and creepily over-doting sister, Benio, seem completely unsurprised when the groom-to-be, Hakuya, and his younger sister, Mashiro, show up on the family's doorstep to move in with them. But it's not until they start attending Kobeni's school that things take an even stranger turn after she starts to pick up some very... unusual vibes from her potential future family. From Mashiro's obsession with aliens and UMAs (Unidentified Mysterious Animals), to Hakuya's disconcerting lack of emotion, there's something very off about them that Kobeni can't quite put her finger on. What has her grandfather gotten her into? And how does this relate to the strange gap in Kobeni's memory? From weird science to the supernatural, almost anything can happen when you're ENGAGED TO THE UNIDENTIFIED.—Sentai Filmworks

Yonomori Kobeni is a high school student who leads a pretty normal life... and then the day of her 16th birthday arrives. Despite having very little presence, her fiance, Mitsumine Hakuya shows up suddenly along with her sister-in-law, a little girl named Mitsumine Mashiro. Without any prior explanation, they begin their peculiar life together. Kobeni's older sister, Yonomori Benio, who has has a sister complex and is a bit of a pervert is thrown into the mix and things get even weirder. Kobeni's life is no longer normal.—Crunchyroll

Not every girl gets what she wants on her 16th birthday, but Kobeni's gift is something she never saw coming: an arranged marriage. Thanks to her late grandfather, Kobeni's been engaged to a total stranger since childhood, and even though she's supposedly met him she doesn't remember a single thing about him. But when her groom-to-be and his younger sister suddenly move in to her house, Kobeni finds that there's something off about these new additions to the family.—Sentai Filmworks