@morter & @Ghost Shogun - lol - I did wonder if they meant, since it's mochi, how can they not be lol?
I guess in this case, her older sister hadn't realised she liked girls till Sasano kissed her just recently (we heard she hadn't had a relationship since she beat up her high school bf lol) - I guess she gave up on relationships and threw herself into work? (It probably failed because he was a guy but she didn't realise?)
And by the point Kasumi was old enough to fall in love, she had already found Acchan.