Miranda: The ProdigalWhen Miranda escaped from her father, she also kidnapped from him her baby sister, a "twin" as she called her. Miranda then hid her from him on Illium. Her mission entails going to Illium and helping Miranda to move her sister and family to a new location. When you get there, you find that mercenaries have discovered her and you fight your way through them to meet up with Miranda's old friend, who you find has betrayed Miranda after learning that she had kidnapped her sister. He will tell Miranda that only he knows about her sister, her father has yet to be informed. Miranda will then shoot him although you have a Paragon option to stop her. After a final fight, you will find her sister unharmed and waiting with her family for a transport. Miranda will want to leave, although you have the option of convincing her to talk to her sister and letting her know she exists and loves her.
From wikipedia-mass effect.