Minutes before the show is set to air, the Academy of Country Music announced that Morgane, 38, will be missing her performance of "Maggie's Song" alongside Stapleton, 43, due to a "prior doula commitment," and will be replaced onstage by Miranda Lambert.
"Due to her prior doula commitment, Morgane Stapleton is unable to join Chris Stapleton at the Bluebird as scheduled," the ACM statement to PEOPLE read. "Miranda Lambert will instead take the Bluebird stage with Chris tonight for 'Maggie's Song.'"
Stapleton later clarified to media in the press room that "My sister-in-law's in labor right now, so my wife was on baby watch — and for once, not her own baby! — but it's been an interesting night tonight so lots to celebrate."
ROBYN BECK/AFP via Getty; Kevin Mazur/Getty Morgane and Chris Stapleton; Miranda Lambert
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Lambert is already set to open the show alongside Elle King for their recently released duet "Drunk (And I Don't Want to Go Home)," as well as perform a second time with Jack Ingram and Jon Randall.
Stapleton's track "Maggie's Song" — written as a tribute to Chris and Morgane's late lab-terrier — appears on his most recently released album Starting Over. It tells the life story of the dog the couple rescued, with Morgane singing background vocals on the track.
"I told her she was a good dog / And I told her goodbye / Run, Maggie, run," he sings on it. "With the heart of a rebel child / Run Maggie, run / Be just as free as you are wild."
"It's all true stuff," Stapleton told the Los Angeles Times about the track. "There's no embellishment in it," before saying, "That one was hard to sing. I kept crying through a lot of it."
Stapleton is the male artist up for the most awards Sunday night, including for entertainer, male artist and album of the year. His song "Starting Over" is also up for song of the year.
Along with the announcement of Morgane missing her performance with her husband, the ACMs revealed that Brothers Osborne will be closing the show with a performance of "Dead Man's Curve." The duo was already scheduled to hit the stage for "I'm Not For Everyone."
The two are nominated for duo of the year, while their album Skeletons is up for album of the year.
The 2021 ACM Awards are airing live from Nashville Sunday from 8-11 p.m. ET on CBS and Paramount+.