The Secret's out!

Tash, Laurie and Sam all came bounding down the hill in their dog forms and transformed back into their usual forms. Unfortunately, Laurie, somehow still had a tail.
"Uuuh, Laurie, you may want to finish off with your tail." I pointed out to Laurie, trying not to laugh.
"Oh," she turned around and chased her tail, "thanks. You know, that's kinda fun!"
"Anyway, Annie, what's this you want to tell us?"
"Yeah! Tell us about your house-change!"
"What? House-Change?" Laurie looked very confused.
"Yeah, House-change because I saw her in Gryffindor common room and in Gryffindor uniform."
"You two, Shut up!!! You," she pointed at me, "Explain!"
"Ok then. I need you to it down. I have a lot to tell you. I'm not an only child as I have led you all to believe, I have a twin sister who is called Sophie."
"HAHAHAHAHAHA!!!!! Good one Annie! Twin sister! Ok, tell us what you're really on about."
"That's the thing. I'm nkot joking, I really do have a twin sister. When we came to Hogwarts, we separated into other factions. I went off on my own, and she went off with some girls whom she had met in Diagon Alley. It turns out that the girls who are her friends are Luella and the
Luella-ettes. None of you say anything. We haven't been in much contact except for some notes. She gets up early to go down to breakfast and leaves early and then she comes in last for lessons and leaves first. She spends her free time outside down in the boathouse with Luella and her clones. Apart from those three friends, she prefers her solitude. She begged me to tell everyone that I was an only child and to only say something if someone saw her. You got that?"
"I think so." They said in perfect unison, all looking at me with the uniform disbelieving expression.
"Ok then. The moment you have been waiting for. Sophie, could you please come out?"

My sister came out from behind the rock which had been keeping her from view.
"Hello. I'm Sophie. Nice to meet you." she turned to me, "Annie, I have to go now. I'm meeting up with my friends down at the boathouse."

The girls all gasped.
When she left, they bombarded me with questions.
"Why didn't you tell us? You promised us you wouldn't keep anything from us."
The girls all left in a huff, all except Laurie.
"You know, I think you were right. You honoured your promise to your sister. I'm guessing the others will be going home for christmas since they both got letters from their parents telling them to come back. I'm staying though. I think it will be fun. Tell you what, we'll map the school together. There won't be anyone to stop us. You think that will be ok? I've got to go, I need to go to the library and finish my H.O.M report on the Goblin revolts of 1287."
"Ok, thanks. See you. Good thing that it's Saturday, the Hols start tomorrow! We must finish our homework so we have holidays free, eh? See ya soon."

I went off towards the owlery to send my parents their Christmas present packages containing my favourite Wizard sweets and a cross-stitch golden-snitch iPod case for mum and a package of Weasley's Wizard Wheezes jokes.

But unknown to me, a sixth person had been listening in on our conversations. Luella came out from behind the little crop of trees and sniggered to herself.
"Hehe, Sophie's going to help me now with humiliaiting Annie. Whether she knows it or not! Muahahaha!!!!"