The King of Queens is a sitcom created by Michael J. Weithorn and David Litt. Starring Kevin James, Leah Remini, Patton Oswalt, and others, the show had nine seasons and aired on CBS from 1998 to 2007.
The show focuses on a working-class couple Doug (James) and Carrie Heffernan (Remini), who live in Rego Park, Queens. The show used to be filmed in front of a live audience and focused on the hilarious situations that the couple used to get in as they lived with Carrie’s widowed and quirky father, Arthur Spooner (Jerry Stiller).
While the show received mixed ratings while it aired, the show gained popularity during syndication. The show has many supporting characters and one of them is Carrie’s sister Sara Spooner, played by Lisa Rieffel.
Here are some details about the character on the show.
What happened to Carrie's sister on The King of Queens?

Lisa Rieffel played the character of Sara Spooner on the sitcom and she was Carrie’s younger half-sister. Sara appeared only in five episodes and upended the lives of Doug and Carrie with her arrival. Sara is someone who is an aspiring but irresponsible actress.
However, one of the biggest mysteries relating to the show focuses on how Sara eventually disappeared from The King of Queens. Interestingly, in the show’s later episodes, it is mentioned that Carrie is an only child. Therefore, the writing off of Sara was an interesting phenomenon.
According to The Direct, an unsourced Wikipedia excerpt mentions that Kevin James had given an interview where he was asked about Sara’s disappearance. The actor replied that her character had to be written off as the producers could not think of any storylines to develop Sara’s character. Therefore, she was discontinued. Even though Sara was heavily present in the pilot episode, her character gets sidelined immensely in the subsequent few episodes where she is present.
More details about why certain sitcom characters get written off

According to The Direct, there are other sitcom characters who are also abruptly written out of the series. Even in The King of Queens, Doug’s friend Richie, a firefighter, is heavily present in the first two seasons but actor Larry Romano exited the show after Season 2 and the character also died.
The Direct mentions that this is called the "Chuck Cunningham Syndrome". It comes from the Happy Days character who was present in the show’s first two seasons but in the later seasons, he simply was not acknowledged. In Seinfeld, the pilot episode features a witty waitress called Claire and she was about to be the show’s female lead. However, she was written off when Elaine Benes came into the picture.
However, actress Lisa Rieffel has gone on to find success after playing Sara and even founded a rock band called Killola. She has also appeared in The Cosby Show, Party of Five, NCIS, and more.
The series is available on Peacock.