Jake Paul insisted Tommy Fury "cracked and tried to find a way out" in withdrawing from their scheduled fight with an injury.
Fury pulled out, and will be replaced by Tyron Woodley on December 18, due to a severe chest infection and a broken rib - he said he was "heartbroken" and hoped to fight Paul in 2022.
YouTuber-turned-undefeated-boxer Paul has lashed out: "For the Furys, it's an embarrassment.
"They said: 'We'd fight Jake with a broken rib, with our arms tied behind our backs'.
"When it came down to fighting, they pulled out."
Paul said about his rival Fury's injury: "I question everything about that family.
"He has never accomplished anything himself. The only reason he is anything is because of his brother.
"He's had a silver spoon in his mouth for his entire life.
"He has got fame from his brother.
"He has never had a real opponent. His opponents were all meant to lose.
"I'm a scary guy to fight - the [trash talk], the pressure. All of this? He cracked and tried to find a way out.
"I truly believe that."
Paul has won all four of his boxing matches since swapping YouTube for the ring.
The younger brother of WBC heavyweight champion Tyson Fury is undefeated in seven.
Asked if Fury's withdrawal gave Paul doubts about rearranging their fight for next year, he said: "It does. He didn't show up to a press conference and now he's pulled out of the fight.
"He has done a terrible job of promoting the fight. He isn't as hype as everyone thought.
"I don't like them. I don't like anything to do with the family. They are sketchy, they are shady.
"I don't see any reason to come back next year and give him a payday.
"It would be more fun to leave him in the dust."
Sky Sports Boxing schedule
Sunday December 12 - Top Rank in New York
Vasiliy Lomachenko vs Richard Commey
Jared Anderson vs Oleksandr Teslenko
Nico Ali Walsh
Sunday December 18 - Top Rank in Montreal
Artur Beterbiev vs Marcus Browne - IBF and WBC light-heavyweight titles
Saturday January 29 - BOXXER in Cardiff
Chris Eubank Jr vs Liam Williams
Claressa Shields vs Ema Kozin - IBF, WBA and WBC middleweight titles
Saturday February 19 - BOXXER in Manchester - Sky Sports Box Office
Amir Khan vs Kell Brook
Saturday February 26 - Top Rank in Glasgow
Josh Taylor vs Jack Catterall - undisputed super-lightweight titles