The sister relationship between Bettyand Polly Cooper. Who are the daughters of Hal and Alice Cooper, and are very close. After being separated by their parents for a long time, the two finally reunited after Betty and Jughead found out where Polly had been forcibly sent by her parents, which is where Betty discovered that her sister was pregnant with Jason's child.
During their childhood, Betty and Polly shared a close bond with one another that remained as they grew up. However, they were separated when, according to their parents, Polly suffered a breakdown due to the collapse of her relationship with Jason Blossom and was subsequently sent to a group home. Their parent then refused to tell Betty where her sister was, that she was too sick for visitors, and told Polly that Betty didn't want to see her.
Since her parents wouldn't tell her where Polly was, Betty enlisted the help of Jughead Jones to help find her. She wanted to see her and also needed to question her about Jason. Betty invited Jughead over for breakfast with her mother one morning and while Jughead distracted Alice, Betty searched her mother's purse and found a check made out to the Sisters of Quiet Mercy, a home for troubled youths.
When she arrived at the group home, she found out that her sister wasn't the crazy person her parents made her out to be, in fact, Polly was pregnant with Jason's baby. This all now made sense to Betty as to why they kept Polly away all those months. Polly explained that she and Jason had planned to run away together, they had a car stashed away ready to go, but Hal and Alice found out and sent her to the Sisters the same day they were meant to leave. It was also revealed that Polly had no idea Jason had been killed and Betty was forced to tell her the truth. Polly and Betty's reunion didn't last long however, as Alice had been notified that Polly had received visitors and arrived to take Betty back home.
As Alice, Betty and Jughead were leaving the facility, they encountered an angry Polly who briefly confronted her mother about Jason. Polly was dragged away by two orderlies but not before Betty made a promise that she would get her sister out of there. That same night, Betty and Jughead found the car of Jason's that Polly had talked about, however, after notifying the Sheriff of it's existence, found someone had set the car on fire while they were away. They then returned to the group home to retrieve Polly only to find she was already gone; escaped through her bedroom window. 
To find Polly, Jughead and Betty organized a search of Eversgreen Forest, which was interrupted by the Blossoms and ultimately unsuccessful. On the walk home from the search with Jughead, Betty realized where her sister was hiding—the attic at the Cooper house. She had hidden there once before when she was little. Betty indeed found Polly, who explained that she couldn't stay at the group home, waiting for Jason to show up since she knew now he never would. Polly and Jason initially had a plan to move to a farm upstate and raise the baby and Polly still wished to go there as their parents were pressuring her to give the baby up for adoption, but she didn't have the money to make it. Polly remained in the attic as Betty tried to think of a solution, and ultimately sought help from the Blossoms. Cliff andPenelope Blossom assured Betty that they would take care of Polly and the baby, however later on, Cheryl revealed that they cared only about the baby and Polly would not be safe with her parents. With no place else to go, Veronica and Hermione Lodge allowed Polly to stay with them.
Free of outside control, Polly was interviewed by Sheriff Keller about Jason and the plans they'd made at the Lodge's apartment at The Pembrooke, Betty by her side. The interview had to be cut short, however, as Polly became upset. Betty comforted her sister and reassured her that she would get through this.
In an effort to help Polly, Veronica suggested to Betty the next morning that they throw her a baby shower so that Polly would feel loved and supported. Betty thought it was an amazing idea and set about organizing it with the Lodges and Jughead's help. They informed Polly of the plan who, while initially unsure, insisted that both Alice and the Blossoms be invited. Betty did her best to convince her mother to attend and to try and mend the rift between her and Polly. If she decided not to attend, however, Betty assured Polly that it would be her loss. Polly then happily surprised her sister as she decided she wanted Betty to be the baby's godmother; Betty was the only person Polly trusted to care for the baby should anything ever happen to her.
At Polly's baby shower, Betty did her best to make sure everything was perfect for her sister, restraining their mother, who they were happy to see had decided to attend, whenever necessary around the Blossoms, who their mother despised. As the event progressed, however, Alice could not help herself and got into an argument with Penelope Blossom which caused Polly to walk off in tears, much to Betty's dismay. Betty told her mother she had to talk to Polly, make things right, and Alice tried to do so, apologizing to Polly. Alice and Betty then tried to convince Polly to return home, but Polly was worried about how their father would react. Betty stated that their dad would just have to accept the fact that Polly and the baby would be returning home. Polly then revealed that Hal had made an appointment for Polly to have an abortion before she had been sent away, which shocked both Betty and Alice.
The next day, Betty assured Polly that their mother had had no idea that their father had made the doctor's appointment and that she had kicked him out of their house after confronting him. Alice wanted Polly and her baby to feel safe, which meant Polly could return home. However, the next morning, despite Betty's best effort to persuade her sister, Polly chose to live with the Blossoms up at Thornhill.