Willie Spears – Jamie's 63-year-old brother who lives in Kentwood, Louisiana – posted an angry, selfie-style video to his Facebook page, urging those who are vocal about their support of the singer to beware.
"I only got one thing to say to all you Britney f--kers out there: Watch what you say now!" he said in the clip, obtained by The Sun.
Despite his aggressive tone and actual message, Willie was seen sitting in front of a wall completely adorned with Britney merchandise and memorabilia.
According to the outlet, he shared another video with similar messaging. His Facebook page appears to have been made private.
Willie previously opened up about the tumultuous dynamic of the Spears family in the 2009 biography, Britney: Inside the Dream, saying that the men often resorted to violence because that's just what they knew.
He told author Steve Dennis at the time, "One time Jamie was drunk and tried to drive off with Britney in the car. She was no more than five years old. I tried to stop him, so I reached into the truck to grab the keys from the ignition and he punched me."
"We got right into it there by the car, fighting in front of her. Britney was jumping up and down, crying," Willie went on, adding that Britney's mother, Lynne Spears, "had to run out to get her inside. We Spears men are known for fighting. If Britney happened to be there, so be it."
Willie recalled how he remembers the exposure to the chaos affecting the then-youngster.
"At first Britney was a scared child and you'd catch her crying but she reached the stage when she just walked off, as if it wasn't happening," he said. "And as she got older, she would scream and curse at her parents to get them to stop fighting."
Willie and Jamie are two of the 10 children their dad, June Austin Spears, fathered with three different women.
Two of their half-siblings – Leigh Ann Spears Wrather and John Mark Spears – recently got candid about the allegedly abusive history of their family, claiming all of the toxicity stemmed from June Austin.
Leigh Ann told The New York Post that while she "almost couldn't believe" what she had heard come out of her niece's mouth in court earlier this summer, it sadly "made sense" to her.
"My mom was like a debutante, she came from a good family and was considered a catch," she said. "But Daddy – I hate even calling him that – drove her crazy. He institutionalized her at Mandeville a few times and put her on lithium. When I heard that Britney was put on lithium for a while I almost couldn't believe it but it made sense. Typical for this family and how they treat their women."
Lithium is the same numbing drug Jamie was accused of administering to his daughter during her 13-year conservatorship.
Leigh Ann and John Mark both described June Austin – who died in 2012 – as a horribly abusive man who allegedly mistreated his kids and wives.
But according to Leigh Ann, she endured more than just physical abuse.
She told the outlet she left home at the age of 16 after allegedly being sexually abused by her dad for five years. She claims that one day, her friend and that friend's mom finally helped her escape.
"I never went back except to visit a few times," Leigh Ann said. "I've been dealing with it my whole life. He used to line us kids up and whip us."
"We'd go to church where my daddy was the deacon and on the way home he'd make me beat him off," she added. "I hope he's burning in hell."
After years of therapy, Leigh Ann said she called her siblings and their spouses to tell them what their father had allegedly done to her when she was a child.
"You know who believed me?" Leigh Ann said. "Only Lynne Spears. I got nothing bad to say about Lynne."
"The Spears men need to be held accountable," she added. "Thank goodness Britney got a good lawyer. Jamie did to her what my daddy did to my mom and [Jamie and Willie's mother] Emma Jean [Spears]. They are mean and they will destroy you if they can't control you. But I think Britney will survive. She's a beautiful person with a beautiful soul."
John Mark echoed his sister's sentiments, telling The Post, "These Spears men are something awful. They've gotten away with so much, especially to their women, for years."
"[June Austin] was a monster but nobody would say a bad word about him outside the family," he added. "Just like no one will say a bad word about Jamie around here [northern Louisiana]."
Like Leigh Ann, John Mark feels June Austin is to blame for the demise of their late mother, Jo Ann Spears, as well as Jamie and Willie's late mother, Emma Jean, who reportedly shot herself on the grave of her infant son when she was just 31.
"He ruined Emma Jean and he ruined my mama," John Mark said. "He shipped them both off to Mandeville [the state's former notorious mental hospital] from time to time. So I'm not too surprised about what Jamie's done to Britney. It's all about control with the Spears men."
Both Leigh Ann and John Mark have doubts about whether Emma Jean's death was a real suicide, noting that "no one knows what happened."
However, says Leigh Ann, "you can bet my daddy pulled the trigger one way or the other even if he wasn't there."
Reps for Jamie never responded to RadarOnline.com's request for comment.