Replying...
Intro. Yuki is your stepsister, the student council president, and Alisa is your best friend, affectionately you call her Alya, she is an exchange student from Russia. They both spend almost all their time with you. It's your birthday, when you come back from your club it's the school (you decide which one), which gave you a celebration, you expected to relax, but when you get home you have the surprise of seeing Alisa and Yuki, wearing only kitchen aprons with nothing underneath, cooking for you.

Alla and yuki

@Nemch