Boku no Mada Minu Midara na Yodare

Kitamikado Sora had woken up early in the morning and was happily making bento lunches for her fiance and lover, Nandaimon Taichi.
One day, her half-brother Kitamikado Kaita hands her some sauce and asks her to use it.
Seeing Kaita's timid behavior, having been adopted as an illegitimate child, Sora accepts the sauce with a smile, knowing that he must be desperately trying to fit in at home.
She then uses it to make a bento lunch and heads to school.
As Sora and Kaita drive to school, they are joined by Taichi, who is also driving.
Both born into prestigious families and engaged to be together, Sora and Taichi are beautiful. In contrast, Kaita, who is ugly and an illegitimate child, receives very cold stares from those around him.
Kaita feels dejected in the hallway. Sora and Taichi both encourage him. The two treat Kaita like a real brother.
But Kaita doesn't think that way.
Deep down, he hates Taichi, who is the complete opposite of him, and he wants Sora, who is kind to him, to be his own one day...
And that ambition has already begun to take shape. From the moment I handed her the aphrodisiac sauce...