"Little did I know I would be relying on my faith, especially as much as I did this past week," Osmond said before dedicating a song to her 18-year-old son, Michael Bryan, whom she called "my angel."
Wrapped in a robe with white feathers that resembled an angel's wings, Osmond genuflected and reached out her right hand as she sang the song amid a starry backdrop.
"How she got through that I will never know," her brother Donny Osmond said after his sister's solo performance.
"I hope you all appreciate what she's going through tonight," he said. "She's a strong woman."
The Osmonds' teary return to the Flamingo Las Vegas on Tuesday night came one day after funeral services for Bryan in Provo, Utah.
Donny Osmond described his sister as strong for simply showing up.
"I just think that we need to give a big round of applause to my sister for even coming onstage tonight," Donny Osmond said after the siblings' opening number.
"Don't do that - I'm going to have to leave," Marie Osmond quipped as the crowd roared in support.
"God bless you all," she said. "The way Osmonds survive is we keep singing and that's what we want to do tonight. I know my son would want that."
The Donny & Marie show resumed before an eager audience that was overwhelmingly supportive of Marie's songs and jokes, as well as a dance-off against her brother, last season's winner of TV's "Dancing With the Stars."
Their sibling banter wasn't spared as they ribbed one another - and themselves - throughout the show about their ages, voices and fitness.
"Thank you, papers, for putting in that I'm 50," Marie Osmond said. "The female F-word - '50."'
Police in Los Angeles have said Bryan died Feb. 26 of an apparent suicide after jumping from the eighth floor of an apartment building. Autopsy and toxicology results are pending.
Police said Bryan left a note but they have not provided details about its contents.
Bryan previously used his adoptive father's last name, Blosil, but Utah court records show his name was legally changed.
At the end of the Osmonds' sentimental return, Marie Osmond cried as she struggled to sing her last line.
"May God keep you in his tender care," Donny Osmond sang, before telling his sister: "You don't have to sing this part."
"'Til he brings us together again," she sang.