Otome Danshi ni Koisuru Otome - Vol. 3 Ch. 276 - Pray for Good Luck

[MangaDex]

 

[Nirimetus]

Oh, that's a good little sister-in-law. Wait, it's his or her sister? lol

 

[Nolonar]

@Nirimetus
They're Kei's siblings.

Chapter 24 introduces Fujieda Kei.
https://mangadex.org/chapter/32129/1

Chapter 220 introduces the Fujieda twins, Fujieda You and Fujieda Rin.
https://mangadex.org/chapter/338917/1