Not to bring anime into this... but a huge amount of the time the "older sister" has to have huge hips and ass and large breasts to separate her from her younger sister. In fact tons of older characters, even 1 or 2 years older, have to have thicker bodies for some reason.
In the rare case that a main/secondary character has smaller breasts than a younger girl it is brought up over and over. But that shit is fucking rare. Sometimes the younger sister is like 18 and the older one is 21 but the younger looks 12 while the older look 35 (with 5 kids).
Like in Nihon they have no idea how women work, even women writers and animators/mangaka don't understand.