The main characters of Brooklyn Nine-Nine were more than a squad. They were basically a family, dedicated to serving and protecting the people around them and each other. While the show mostly focused on their lives in the 99th precinct and in the field, the audience got to see their family life as well.
As with every other episode involving only the main characters, their family's appearances on the show brought a lot of laughter and entertainment. There were good and fun relatives, as well as terrible ones, and some who were kind of in-between. Even though a number of their relatives only appeared or were spoken of a handful of times, they made quite an impact in that short time; enough to make a lasting impression as some of the worst relatives on Brooklyn Nine-Nine.
Eleanor Horstweil
Based on comments Charles made throughout the show and Eleanor's appearance in "Hostage Situation," she always bullied him during their marriage and after their divorce. The horrific things she did to him ranged from making him get a vasectomy to having him rent out her basement when she divorced him and got re-married, which was both sad and tragic.
When Charles finally moved out and on with Genevive, he decided to unfreeze his sperm so they could start a family. Unfortunately, due to a contract Eleanor made him sign, she had full control of all his frozen samples and withheld them, demanding that he use his authority as a cop to threaten a priest she'd run over to drop the charges in return for his sperm. She even dumped one of the sample cups down the garbage disposal to prove how serious she was.
Zeke
Terry's brother-in-law appeared on Brooklyn Nine-Nine three times, and each time he subjected Terry to condescending and demeaning remarks. Zeke was actually bigger in size than Terry, which was quite something considering one of the show's running gags was Terry's size and muscles. Because of this, Zeke nicknamed him Tiny Terry and taunted him relentlessly.
Whenever he was visiting, Terry avoided going home early so he wouldn't have to hang around him much. On one occasion, he and Captain Holt had no choice but to go back home and work from there and Zeke and Terry actually bonded when he thought Holt was a mean boss like his. In season three, Zeke attacked Terry for leaving the dinner table during the Christmas holidays for a work emergency and Terry finally snapped and yelled at him to get a job and stop hounding him over his.
Roger Peralta
Jake's issues as a result of his strenuous relationship with his father were touched upon several times throughout the show, including how Jake had witnessed him cheating on his mother when he was younger. When Roger finally appeared on the show in season 2's "Captain Peralta," he lived up to his reputation.
Under the guise of spending time with Jake, Roger used him to get cleared of drug trafficking charges. He also got back together with Jake's mum, which Jake wasn't happy about, given their history. Although their relationship improved and Jake made significant progress towards healing from his childhood scars, the negative impact his father had on him is still a defining part of his character.
Kate Peralta
Jake discovered he had three half-sisters and he met one of them in "DFW." When he and Amy first met her, she was handcuffed and being frog-marched by police for vaping on the plane and "slapping a stewardess with her fist." It all went downhill from there.
They went for dinner at a restaurant, where she offered to pay for the meal. To Jake and Amy's horror, she pulled out a bag of glass and planted some in her food then made a scene, claiming the food came with glass in it so she wasn't going to pay. She even tried to get her return ticket upgraded to business class by cutting her hand on glass from her bag and claiming she cut it on a ticketing kiosk. Once he got to know and understand her better, they made peace, but she made no promises to his request for her not to vape on the plane again.
David Santiago
In an episode aptly titled "The Golden Child," fans got to see David, one of Amy's seven brothers and biggest rival because he was their parent's favorite. Throughout the episode, he flaunted his achievements and skills to Amy and Jake. Their mother made it worse by openly showing that David was her favorite, which is possibly the reason why Amy is so competitive.
Even when Jake tried to hype Amy up to her mom, Camila countered each of Amy's achievements with something better that David had done. During the episode, David was arrested for drug possession, which Amy was happy about as it proved he wasn't perfect until he revealed it was all a setup. By the end, Amy realized it didn't matter if David was better at everything.
Camila Santiago
Camila Santiago didn't make any effort to hide that Amy's brother, David, was her favorite. According to Amy, her parents arranged pictures of their children according to whoever made them proudest, with the prime spot being the mantle where David's picture was always displayed.
Apart from this, she was a little patronizing and judgemental, especially during the Thanksgiving lunch with Jake's parents where she brought her own turkey even though she was a guest. During a rant in "The Golden Child," Jake also mentioned how Camila brought her own coffee when she visited them and made fun of his nose.
Kevin's Parents
Kevin's parents didn't make an appearance in an episode of Brooklyn Nine-Nine, but Captain Holt mentioned them in the episode "Captain Peralta." Although he only said one thing about them, it was something big.
While Jake prepared to meet and impress Amy's father, Captain Holt advised him not to be overconfident and gave an example of a blunder he'd made when he met Kevin's parents. He also said, "they're huge homophobes who think that he made Kevin gay" and haven't spoken to him since - a truly appalling story.
The Diazs'
When Rosa came out to Oscar and Julia, they didn't take it well. At first, they refused to accept it, preferring to believe Rosa's lie about an affair with Jake over her being bisexual. Later in the episode, they invited Rosa over for their traditional family game night and Rosa happily went, believing it to be a sign of their acceptance.
During game night, Julia said she didn't care what Rosa called herself, as long as she married a man at the end of the day, while Oscar said it was a passing phase and she'd definitely marry a man - biphobic comments that clearly hurt Rosa. Oscar came around by the end of the episode and apologized, but Julia needed more time, which meant Rosa couldn't go to family game night for a while. Julia did, however, eventually begin to accept Rosa in later seasons.
Gintas Irbe
Gintas showed up in the tenth episode of season 6, "Gintars" when he stalked Jake and Charles before telling them he was Nikolaj's biological father and wanted to meet his son. As Nikolaj's adoptive father, Charles wasn't happy about it and denied the request. He only changed his mind when Jake suggested that Gintars meet Nikolaj without revealing the truth about him being his biological father.
Gintas went against their deal and told Nikolaj he was his father, completely disregarding how disruptive such an out-of-the-blue revelation would be for a child. Besides that, Gintars owned and operated a knockoff denim brand called GAPE.
Earl Scully
During a heist in "Cinco de Mayo," Jake brought in Earl—Norm Scully's identical twin—to trick the others. His plan backfired when Norm discovered Earl was in the building. As it turned out, Earl slept with Norm's wife in the past, causing a rift between them.
When Norm reminded him of that, Earl mocked him and said Norm's wife had cheated because she wanted to be with a stud instead of a dud. A scuffle followed, but they eventually made peace at the end of the episode, thanks to Hitchcock's advice.