Per Andrew Marchand of the New York Post, Peyton didn't want to be in a position where he would have to be critical of Eli during a game.
"Manning's desire was so strong there was even talk, according to sources, that if Manning had agreed to do Thursday night games, Fox would try to make it so the Giants did not show up on its primetime schedule," Marchand wrote.
Manning had offers from Fox, which will start broadcasting Thursday night games this season, and ESPN, which had an opening after Jon Gruden was hired as head coach of the Oakland Raiders, to serve as a color commentator for their coverage of NFL games during the 2018 season.
Marchand reported last month, however, that the five-time NFL MVP turned down both networks.
John Ourand of the Sports Business Journal said on the April 4 episode of Good Morning Football that Manning has "no interest" in becoming a sports broadcaster.
Marchand did note calling Eli's games wasn't the only factor in Peyton's choice, but "it was part of the process."
Since retiring from the NFL after the 2015 season, Manning has made sporadic appearances at NFL games. He was honored by the Indianapolis Colts during their Week 5 game against the San Francisco 49ers last season when his No. 18 jersey was retired at halftime.