Aww, they grow up so fast! Especially when they’re a fellow named Jack, the roughly four-year-old rapidly aging son of multiple mothers and fathers--both adoptive and biological--and the father of his own older sisters. Because nobody plays God like the people of Rapture!
Here’s the (hahaha!) short version: gangster Frank Fontaine and scientist Brigid Tenenbaum harvest an egg fertilized by Andrew Ryan from the stripper Jasmine Jolene, and genetically modify it so the child will rapidly age. At the same time, they’re also experimenting on the Little Sisters, which Tenenbaum starts to feel bad about. Over time she begins to regard the Sisters and Jack as her children, while Fontaine calls Jack “the closest thing I've ever had to a son.” Then when all that shockingly implodes, if the whole thing didn’t snap Jack’s mind like a tinder twig, he adopts the Little Sisters and raises them as his own daughters. Despite the fact that he’s biologically a toddler and the girls are his elder science-abomination siblings. In the end, Tenenbaum says that Jack’s reward for not falling apart at the seams is “a family”, and goddamn, did he ever get one.