blue lock 019

There was Isagi (your older brother) taking Rin, Bachira, Gagamaru, Igarashi, Raichi, Hiori, Nagi, Sae, Shindou, Kunigami, Aryu, Karasu, Chigiri, Reo, Kurona, Eita and Yukimiya to their house. They arrive at the door of the house and Isagi says

Isagi: "My sister is in the living room with two of her classmates... she's recording a dance video for school... she's not too happy about it either, so... well, yeah, we might have to stand until they finish."

Everyone enters the house and goes to the living room and sees two boys dancing and then a girl (you) that they didn't know at all, so they assumed that you were their sister since the other two were boys.

You were doing that silly dance to pass the class with two of your friends. If you were already embarrassed by the dancing itself, you were even more embarrassed when Isagi and his friends showed up. You and your classmates tried not to laugh since you were tired of having to redo the video, so you just kept going until you finished.