They go to the bathroom together every day.

They both suddenly disappear during lunchbreak, so it's diffcult to find them

Occasionally, some students mistake Mio for the older sister and Mayu for the younger sister.

They're always together, like Tandemaus

The younger sister can at least make small talk, but it's not so simple with the older sister.

Surely, there were talks that only one of them was feeling sick, but they both ended up leaving early together.

There are probably mob characters who like both of them (Jealousy-provoking bait)