Denning Sisters

The Denning Sisters, Barbara and Carol, were the primary antagonists of the sixth season of the Netflix series Orange is the New Black. They were both long-term residents of the Litchfield Maximum Security Prison.

Background
Barbara and Carol Denning were the eldest and second-eldest respectively of three sisters, the youngest of the three being Debbie, a talented dancer. Partway through what would have been Barbara's high school graduation year, and Carol's second-last year, their parents decided to relocate to Texas in order to send Debbie to a prestigious dancing academy, despite the objections of Barbara and Carol. Eventually, the two elder sisters conspired to murder Debbie and make her death look like an accident. Despite Debbie confessing that she didn't actually want to go to Texas either, this did not save her life, and her sisters locked her in the family car then pushed it into a frozen lake, killing Debbie. Their attempts to make her death look like an accident were found out almost immediately, however, forcing them to go on the run for a while, before they were caught and sentenced to life in prison.

When they arrived at Litchfield, the sisters were assigned separate cell blocks, the "C" block in Carol's case and the "D" block in Barbara's case. Because of the strong rivalry and mutual hatred between the two blocks, the relationship between the sisters rapidly deteriorated, with both taking on followers and starting rival contraband supply businesses. The situation started to deteriorate when Carol's supply of items suddenly went missing, and her right-hand woman, Frieda Berlin persuaded her that Barbara must have been responsible, and that she should retaliate by having her followers attack Barbara's during a kickball game.

Sure enough, Carol and her followers attacked Barbara and her own followers with a variety of improvised weapons, with several on both sides being severely injured and even killed. In the middle of the fight, however, Barbara suddenly claimed that it was Carol who had stolen her stash, not the other way around, causing the two sisters to furiously realize that Frida had played them both. By then it was too late to get revenge on Frida, as she had been transferred to the nearby minimum-security prison as part of a deal with the warden, and both sisters subsequently received a range of additional sentences for smuggling and their part in the brawl, ending any remote hope of them ever getting out of prison. To add insult to injury, the incident also got open-air sports banned at the prison, robbing them of one of the few pleasures they still had in their lives.

In the decades that followed, the sisters fostered a sense of hatred between their respective blocks, but never allowed it to explode into open warfare. That all changed when Frida, along with almost the entire population of the minimum-security prison, was sent to the maximum-security facility as punishment for their part in a riot. Due to her advanced age, Frida was put in "B" block, the most plush of the four prison blocks, putting her just out of reach of both sisters. They therefore called an uneasy truce (while maintaining the appearance of rivalry to their gang members) as part of a plot to get revenge on Frida, and when one of the other new inmates, Piper Chapman suggested bringing back kickball games, the two sisters hatched a plot and encouraged their followers to sign up, each telling their respective followers that it would be a perfect excuse to kill their rivals.

While there were various speedbumps along the way, including Barbara's drug route into the prison collapsing and the resulting withdrawal symptoms among her followers resulting in a brief outbreak of violence that caused security to to stepped up, along with Carol's new right-hand woman Madison Murphy trying to get the kickball game called off due to her hatred of Piper, the day of the game finally arrived. The two sisters hid together in a closet and revealed that their actual plan was to disguise themselves as "B" block inmates, then sneak into that block during the security lockdown that would follow the planned brawl between the kickball players, allowing them to get revenge on Frida once and for all.

In the end, however, their plan fell apart when the former minimum-security prisoners, who had never bought into the rivalry between "C" and "D" blocks, managed to talk their fellow block members out of going through with the planned attacks. The two sisters, not knowing what had happened, soon accused each other of betrayal, and an argument over which of the two sisters an amusing anecdote involving someone mixing up the phrases "classifieds" and "glass of ice" had happened to (which in reality wasn't either sister, but rather a waitress they worked with while on the run) ended up resulting in them stabbing each other to death, with Frida remaining blissfully unaware of their plot against her.

Personalities
As a teenager and a young woman, Carol was the more openly psychopathic of the two sisters, prone to frequent violent outbursts, and openly fantasizing about killing people she didn't like, which she often made illustrations of in her scrapbook. By contrast, Barbara was a little more controlled, but eventually snapped due to the combination of parental favoritism towards Debbie and the implied abusive behavior of their father, resulting in her agreeing to help carry out what was indicated as Carol's plan to kill Debbie.

During the story's present-day, however, this had reversed itself somewhat, with Carol having become a little more controlled and calculating, and Barbara more paranoid and unstable from her frequent drug usage. Though the break in Barbara's drug supply helped her stabilize a little, the two sisters remained unstable and distrustful towards each other, resulting in their ultimate demise.

Though they outwardly appeared protective of their gang members, in reality they viewed them all as expendable, and were happy to cast off anyone if they became inconvenient or a more useful person showed up.