"Harry, good to see you, please, do sit down." Dumbledore gestured to the one vacant chair across from his desk. Harry dropped into it, somewhat reluctantly, crossing his arms over his chest and frowning at the headmaster. Snape and Malfoy were seated off to one side, while Sirius was curled up, in dog form, on the floor at Harry's feet. Dumbledore fiddled with his half moon spectacles for a moment, before returning them to the bridge of his nose and clearing his throat.
"Harry, something has come up. Voldemort has reason to believe that you're sister is still alive, living in the States, he-" Harry jumped up from his seat.
"Sister? I have a sister? Why did no one bother to tell me this?" he shouted. Dumbledore leaned back in his chair, peering at the boy.
"We had no reason to believe she was still alive. No one has seen her since the night of your parents' deaths. We had believed he killed her then, as well." He explained, trying to calm Harry enough for him to see the reasons behind the Order withholding the information.
"So did you ever plan on telling me about her?" Harry asked, leaning his palms on the desk in front of him.
"Why the hell not? Do you think that makes anything better? Keeping things from me? Well. What's her name?" he asked, defeated, slumping back down into his seat.
"Lily." Draco whispered. For a moment, Harry had forgotten that he and Snape were there. "Lily Marie Potter."
"She looks just like your mother, but she has your father's eyes. Believe it or not, you are twins. The prophecy applies to her, as well. That's why we need you to come with us to retrieve her. She's aware of her magic, and has been attending a wizarding school in America. Midaflan School for witches, if I'm not mistaken." Snape said, leaning his elbows on his knees.
"Voldemort wants her. If he can create heirs with her, he believes that will resolve a part of the prophecy, though he still believes he needs to kill you. We need to prevent that from ever happening." Draco said. He stood up from his chair and started pacing the length of the office.
"When do we leave?" Harry asked, joining Malfoy.
"As soon as possible, we just need you to go get your cloak." Dumbledore spoke up. Harry nodded and left the office, running for the Gryffindor tower.
He shouted the password to the fat lady, and shot up the stairs to the boys dorm, ignoring his friends shouts and questions. He knocked his trunk onto it's side, letting the contents spill out onto the floor. He spotted the invisibility cloak easily, shrunk it, and stuffed it into his pocket, before charging back down to the common room and out the portrait hole. Snape, Draco, Dumbledore and Sirius were waiting for him in the entrance hall, wands at the ready.
"I will wait here, for your return, until then, good luck. You will need to perform side-long apparation, Draco with Snape, Mr. Potter with Sirius. One of you will have to apparate back on your own, Mr. Malfoy, preferably. I doubt Lily has learned how to, completely, that is. Once again, good luck." With that final parting, Sirius grabbed hold of harry and disapparated with a loud CRACK.