the concept of this story is a little confusing to me i like yuri but if they’re role playing as an older sister and younger sister it feels very incestious. The side couple honestly has a more interesting dynamic than the main couple.
Class S relationships were what Yuri was in a world where Yuri was not acceptable.
It's an early 20th century concept in Japan - intense emotional and romantic friendships between girls, often young adults or teenage schoolgirls.
It was viewed as a "phase" for young girls to experience these relationships, before coming of age and getting married.
The sisterly affectations is largely about implying this lesser status versus a true same-sex love.
It was a popular genre of fiction until it was banned in Japan in the 1930s.