Vitali and Wladimir Klitschko are worried that promoters will deny them the chance of holding all four heavyweight titles, AP reported.
The Ukrainian brothers currently hold three of the belts — Wladmir is the IBF and WBO champion, and Vitali is the WBC champ.
Only WBA champion Nikolai Valuev stands between a historic four-belt family sweep.
The Klitschko brothers claimed Monday that Valuev`s promoters, Don King and German Wilfried Sauerland, are holding up the chance for one of them to take his title by offering an unacceptable share of the purse.
"Around the world fans and experts want to see this fight," Vitali Klitschko said. "But these promoters are doing everything to make sure that this fight doesn`t go ahead."
Vitali Klitschko said the purse should be split 50-50 because it would pit champion against champion.
Both Vitali, 37, and Wladimir, 32, said they would be ready to fight the 7-foot Russian, and Valuev himself has appeared agreeable to the idea.
"The most important thing is to bring the belt to the family," Wladimir Klitschko said.
If the fight does go ahead, Wladimir Klitschko said, it will likely be toward the end of the year. That might be too late for the brothers to achieve their dream of holding all four belts at one time.
Vitali Klitschko, who announced his retirement in 2005 but came back in 2007, is preparing for the mandatory defense of his WBC title against Juan Carlos Gomez of Cuba on March 21.
He had hoped to fight Britain`s David Haye in June in London, but will instead take on the WBC`s No. 1-ranked challenger in Stuttgart.
Wladimir Klitschko said he would now likely face Haye, though arrangements have just begun, and mentioned Chris Arreola as another possible opponent this year.
The brothers reiterated they would never break a promise to their mother and square off against each other in the ring.
"Our mom will not survive this fight, and we have to take care of her," Wladimir Klitschko said.