
i think the big sis is not homophobic more like she is just broke up with her boyfriend it just she dont like to see loveydovey couples

It was the friend that broke up with her boyfriend, not the sister.
http://www.mangago.zone/read-manga/the_best_smell/mh/ibx_chapter-70/pg-1/

http://www.mangago.zone/read-manga/the_best_smell/mh/ibx_chapter-69/pg-19/
Well it looks like they're also referring to the sister's boyfriend too. The next few panels also further discusses the "scum" and the sister looks angry too. Maybe they both broke up with their respective boyfriends.
i miss them