idk if there's any confirmation from Kuromasu, but Yuna here is referring to Erina by her name only, doing 呼び捨て. That makes me think the lil' gyaru is her younger sister.

TokimekiPoporon said:

idk if there's any confirmation from Kuromasu, but Yuna here is referring to Erina by her name only, doing 呼び捨て. That makes me think the lil' gyaru is her younger sister.

Now confirmed in post #8382405.