In the first of the Bioshock franchise you learn of a little girl named Masha Lutz who had been taken from her family by Andrew Ryan’s men and turned into a little sister by Dr. Suchong’s conditioning. Her parents then committed suicide in despair when they realized she no longer knew them.
In Bioshock Infinite : Burial At Sea Ep. 2, you meet two little girls inside of Dr. Suchong’s Free Clinic. The two girls are named Leta and Masha. They are seen cowering in fear from an injured Bouncer, which they refer to as “Monster”. Once Elizabeth reveals that the Big Daddy will die without their ADAM, the two girls feel sorry for the monster, believing that it’s unfair he’s going to die. Leta then injects him with some of Masha’s ADAM-rich blood, healing him. The Big Daddy then adopts the two girls, completing the bond that Suchong had been working so hard to create.
(Leta on the left and Masha on the right)