The girl who died Harry Potter's Twin Sister

Chapter 2- HBP

"Cissy wait!" Bellatrix followed, her cloak streaming behind and saw Narcissa darting through an alley between the houses into a second, almost identical street. Some of the street lamps were broken. Bella managed to grab Narcissa's arm as they turned another barley lit corner. "Cissy you must not do this, he cannot be trusted-" 

"The Dark Lord trusts him- doesn't he?"

"I fear the Dark Lord is... mistaken," Bella panted and her eyes gleamed momentarily. "In any case, we were told not to speak the plan to anyone. This is a betrayal of the Dark Lords-"

"Let go, Bella!" Snarled Narcissa. 

Narcissa rushed ahead and turned another corner and on to a street called 'Spinner's End' 

She had knocked on the door before Bella, cursing under her breath, had caught up. Together they stood waiting, panting slightly, breathing in the smell of the dirty river that was carried to them on the night's breeze. After a few seconds, they heard movement behind the door and it opened a crack. Silver of a man could be seen looking out at them a man with long black hair parted in curtains around a sallow face and onyx eyes. 

Narcissa threw back her hood. She was so pale that she seemed to shine in the darkness, the long blonde hair streaming down her back gave her the looked of a drowned person. 

"Narcissa!" said the man, opening the door a little wider so that the light fell upon her and her sister too. "What a pleasant surprise!" 

"Severus," she said in a strained whisper. "May I speak to you? It's urgent." 

"But of course." He stood back to allow her to pass him into the house. Her still-hooded sister followed without invitation. 

"Snape," she said curtly as she passed him. 

"Bellatrix," he replied, his thin mouth curling into a slightly mocking smile as he closed the door with a snap behind him. 

They had stepped directly into a tiny sitting room, which had the feeling of a dark padded cell. The walls were completely covered in books, most of them bound in old black or brown leather, a threadbare sofa and a small table. The place had an air of neglect, as though it were not usually inhabited.

"Now... You came to ask me for help, Narcissa?" Narcissa looked up at him, her face eloquent with despair. "Yes, Severus. I think you are the only one who can help me, I have nowhere else to turn. Lucius in jail and...

She closed her eyes and two large tears seeped from beneath her eyelids. "The Dark Lord has forbidden me to speak of it," Narcissa continued, her eyes still closed. "He wishes none to know of the plan. It is... very secret but-"

"If he has forbidden it, you ought not to speak," said Snape at once. "The dark lord's words are in law." Narcissa gasped as though cold water had been poured on her. Bellatrix looked satisfied for the first time since she has entered the house. "There! Even Snape says so!" She said loudly to her sister.

"It so happens I know of the plan," he said in a low voice. "I am one of the few the Dark Lord trusts." 

"It might be possible for me to help Draco," She sat up, her face paper-white, her eyes huge. 

Bellatrix stood with her mouth wide open as Severus and Narcissa clasped hands to make the unbreakable vow. 

"Will you, Severus, watch over my son Draco as he attempts to fulfil the Dark Lord's wishes?" 

"I will," said Snape. 

"And, should Draco fail, will you carry out the Dark Lord's orders?" There was a moment of silence, Bellatrix watched intently, her eyes wide. 

"I will," Snape finally spoke. Bellatrix smirked and walked out. Just as Snape was about to let go, Narcissa tightened her girl. 

"And the Potter girl, Will you protect her?" Snape rose an eyebrow but said, "I will." A flame twisted and bound itself thickly around their hands, like a rope, like a fiery snake. They let go. 

"What is the Dark Lords wishes with Elizabeth Potter?" 

"He knows of her powers. He wants her, she can help him defeat Harry Potter. I have to go Severus." Snape nodded and saw Bellatrix and Narcissa out. 

How does The Dark Lord know of her Powers?