A/N No romantic Blackcest intentional, just plain old sisterly caring
Bellatrix immediately got up and started pacing. The dark mark hadn't vanished when the Dark Lord had disappeared the first time. So what, he must really be gone this time? Bellatrix scoffed, that was highly unlikely. She stopped suddenly, that would mean she could go home… She could see her sisters again.
Memories of her last night as the Dark Lord's second in command overwhelmed her with all the force of a tidal wave.
"You have to go" Andromeda shoved her sister towards the door "Now, before he comes back"
Bellatrix tried to stand her ground pushing back weakly, exhausted from all the fighting. Why did they always have to fight… "He will come back for me."
"Bella" tears fell freely down Andromeda's face, she couldn't lose another person she loved. "Please..." Ted was already dead, the first target of the so-called Dark Lord on this new tirade. What made it worse, was that it had been his reward for Bella. Little did he know, that although Bellatrix loved Tom, she also loved her family even though she had to keep their contact secret for so many years.
"He will come" Bellatrix was still struggling against her sister's helpful hand, "Join us" Her eye lit up, maybe they could have it all… be reunited! "I will talk to the Dark Lord, yes… I will make him see that you never loved the mudblood, that you're no blood traitor!"
Andromeda hung her head, and stopped fighting for a moment. She pulled her sister into a fierce hug, putting into it all the things she was too proud and too stubborn to say.
"He will lose" It was barely a whisper.
Bellatrix recoiled but still held Andromeda, albeit at arms length. Capturing her sister's gaze, she pleaded silently for her to listen. Once the Dark Lord killed the boy, he would be invincible, she needed her sister to join him now, she needed to keep her sister this time and most importantly she needed to keep her baby sister Andromeda alive.
Bellatrix didn't understand. Andromeda made a terrible split second decision.
"Yes"
"Yes?!" Bellatrix knew her sister better than to think she had given up so easily "what's the catch?"
"Make the Unbreakable Vow with me"
Bellatrix's face would have been comical, eyes wide and mouth agape, if not for the severity of the situation and the smudged tear lines that streaked down her otherwise grubby face.
A/N The Bella/Tom thing is not really a plot point, (and there will most definitely be nothing explicit) never fear Bellamione shippers!