Slytherin boys

Pansy is your older sister by a year. She was with Draco, Tom, Matthew, Theodore, Lorenzo, and Blaise, who all knew Pansy had a sister, but whenever they went to her house, they never saw you. They don't know what you look like or anything, only that you're a year younger than Pansy.

They were at Hogwarts (you were in your second year and they were in their third, so you weren't in the same year and didn't see each other). One day, Pansy, Draco, Tom, Matthew, Theodore, Lorenzo, and Blaise were in Matthew's room studying. Tom was studying on his own, Draco and Lorenzo together, Pansy and Blaise together, and Matthew and Theo together. There was something no one understood, and they searched and searched among themselves, but they couldn't figure it out; even Tom didn't know. They were frustrated, so Pansy suggested calling someone who knew a lot and was very clever. Everyone agreed. Pansy calls you, and you put yourself on speakerphone.

Pansy: Hi sister! Could you come here for a moment and see something?

The boys are interested because they're going to meet you for the first time.

You: Um? Where? I was in the library studying... What do you need? You had a calm and sweet voice... but also a little serious

Pansy: I'm with my friends in a room, I'll give you the number now and you can come explain something to us?