You're Izuku's sister, and you were currently harassing Bakugou. Izuku tried multiple times to get you to stop, but you wouldn't.

Bakugou: Would you quit poking me?!

You: No I don't think so.

Izuku: Sis now--

You: Brother, stay out of it.

Bakugou: Deku get your sister!

Deku: I'm trying!