Two of Litchfield maximum security's most daunting figures take their time making their way to the spotlight, but when they do, their influence is palpable. But until late in the season, their backstories are both unclear. So why are Barb and Carol in prison on OITNB? Spoilers ahead for the new season of OITNB.
The animosity between cell blocks C and D was established the moment Orange Is The New Black's character walked into their new, foreign home. And the root of all of it, it seems, are Carol and Barb. In an Episode 10 flashback, viewers finally realize that the connection the two share is deeper than just that of prison rivals — they're actually sisters who committed a very grisly crime.
The flashback shows them as teenagers, agonizing over the fact that their parents are uprooting their family to move their younger sister, Debbie, closer to more elite dancing training. It seems very clear that their parents are much more invested in the youngest daughter than they are with Barb or Carol, and the older sisters know it. They are sick of playing second and third fiddles to the child, and even though the girl has never personally done anything to them other than steal away their parents' affections, Barb and Carol take matters in to their own hands.
One night, when the sisters are all three left alone to finish packing up their home, Barb and Carol take Debbie to release some tadpoles she's been raising. After arriving to the icy lake, releasing the tadpoles, and having a seemingly warm moment with her little sister, Barb turns around to walk her back to their parked car. At that moment, Carol runs at Debbie with an ax, and eventually they corner her in a locked car.
Unable to escape, the sister screams and cries as Barb and Carol almost gleefully shove her into the body of water, and watch as the car submerges into the depths. The aftermath in regards to their parents, their trial, or anything else is unclear, but they're definitely caught and prosecuted, because the next flashback takes place during their early years in prison.
Viewers aren't given a ton of background in regards to Barb and Carol's upbringings aside from the fact that their relationships with their parents are obviously strained, so other factors could definitely be at play causing them to be so callous. But regardless, it's downright chilling how casual and blasé the two are about killing their sister. It's a decision they come to almost immediately, and apparently one they become famous for.
Frieda tells Suzanne later in the episode that they became known as the "Little Debbie Murderers," so their case obviously garnered some kind of recognition in the true crime world or in the press. Frieda also says that the reason the cell blocks are constantly at war is because Barb and Carol turned on each other in prison, were separated into different blocks, and subsequently gathered supporters behind each of them.
Their criminal backgrounds also just make their presence in Litchfield even more menacing than it was from the beginning. Upon learning about the murder of their sister, it's impossible for viewers to imagine what else they could potentially be capable of doing to anyone else who remotely gets in their way. All the kid wanted to do was be a gymnast and raise some tadpoles, you know?