So for additional context, Ayane's cousin (and their backstory) is elaborated in a
spinoff. But seeing as it's
not translated in English yet, I'll try giving a summary. Tho trigger warning, it kinda touch on a controversial topic (sexual harassment /
implied grooming). So if you're uncomfortable with it, don't reveal the spoiler.
Basically, when she is in highschool (but before the main story start), Ayane's parent is always busy and so she was often left alone in her home. Sometimes, her older cousin (Sachi) would come to accompany her. Ayane used to like her, tho she herself doesn't know whether it's platonic or romantic.
Sachi noticed that Ayane is fond of her, and one day she ask Ayane is she has a crush. Ayane answer vaguely, and then Sachi ask Ayane if she would like to 'practice' with her so that she wouldn't be 'clueless' when she does end up with a lover. By this point, you all can probably imagines what happen next.
It's implied that Ayane become a good 'top' (as testified by Rika) because she was emulating what Sachi did to her.