Trainer Floyd Mayweather Sr has swapped camps for the May showdown between his son, Floyd Jr, and Oscar de la Hoya.
The two Mayweathers had been estranged for years, with Floyd Sr employed as De la Hoya's most recent cornerman.
But De la Hoya replaced Mayweather with LA-based Freddie Roach in a disagreement over his trainer's fee.
And in a stunning new development, Mayweather Sr appeared at the latest fight media conference to reveal he has now become part of his son's team.
"He'll be in the corner," Mayweather Jr told De la Hoya. "But not your corner."
"If this fight comes down to us going to war, I'm going to ride with my father"
The Mayweather reunion occurred just 30 minutes earlier in a suite upstairs at the MGM Grand Hotel, where one of the most anticipated fights in years takes place on 5 May.
Mayweather Jr has been trained by his uncle, Roger Mayweather, but he is currently in jail on domestic abuse charges, though he is expected to be released before the fight.
"We have our ups and downs, but he's still my father," Mayweather said of Floyd Sr.
"And if this fight comes down to us going to war, I'm going to ride with my father."
Floyd Sr laid into his former employers and mocked Roach's reputation as a coach.
But De la Hoya remained passive and quipped: "I' m just really glad I can bring family together like it's supposed to be."
De la Hoya, 38-4 with 30 knockouts, will defend his WBC light middleweight crown against WBC welterweight champion Mayweather (37-0 with 24 knockouts) in Vegas.
Mayweather Sr told BBC Sport in January he wanted $2m to be in De la Hoya's corner.
He says De la Hoya only offered him $500,000 (£253,000), with an additional $500,000 if he wins, an offer Mayweather Sr found unacceptable.
The fight between the two Americans could rival records for pay-per-view and live gate income with ringside seats going for $2,000 (£1,020) each.
The Nevada record for paying fans is the $16.8m (£8.57m) spent by a sell-out crowd for Lennox Lewis' heavyweight unification triumph over Evander Holyfield in Las Vegas in 1999.
De la Hoya, 33, last fought in May when he stopped Ricardo Mayorga in six rounds to win the title.
Mayweather, 29, defeated Carlos Baldomir in a welterweight title bout in November.