Draco once had a heart and she once had a soul. Then she left. Now she's back. She's sarcastic and doesn't give a stuff about anyone. Life sucks when your past returns, doesn't it.
Only three people knew that Harry had younger twin sisters hiding from Voldermort. Now that Voldermort is back, the sisters come back as well. Hogwarts get turned upside down with the opposite personality sisters. I know there a alot of stories like this