The famous siblings began their Vegas residency in 2008.
On the Watch What Happens Live aftershow Tuesday night, the 59-year-old singer said the nearly 11-year-long residency will probably play its final show this year.
"We say it every year and then we do another year," Marie confessed. "We're kind of thinking this year. We decide 10 years and then you turn it to 11. And so we're just thinking maybe this year, but I don't know."
"Donny's getting old," she quipped of her 61-year-old brother. "It's really fun."
Donny & Marie at the Flamingo Hotel kicked off in 2008 and was initially scheduled for a six-week run. Back in October, Donny told Las Vegas Morning Blend that 2019 would mark their final year. The show's website currently shows Nov. 16, 2019, as the last day to purchase tickets.
“Next year’s our final year,” he said at the time. “We’ve been talking about it for a long time, but definitely, our countdown has started.”
On the WWHL aftershow, Marie confirmed Donny's comments. "Donny has said this many years," she noted. "I don't know. Honestly, we are thinking this year. We were supposed to be there six weeks."
Prior to the start of their residency, the siblings hadn't worked together for nearly 30 years. Returning to the stage together, Marie revealed, had its weird moments.
"We kind of looked at each other and went, 'Wow, you're older,'" she joked. "But we thought, six weeks, we can handle that. And here it is, 11 years [later]. Now we're moving into that."
"But it's fun and people come from all over the world," she added. "... It's an interesting room. There's lots of rooms in Vegas, but it's one of the old school rooms when you're in the booths and you're really close. When you have that kind of a room, you do a different kind of a show. Instead of having a stage with 40,000 dancers on it and everything. It's been fun."
Earlier in the night, Marie revealed whom she considers to be her most talented brother in response to a caller's question. Marie has eight brothers, but the original barbershop quartet included Alan, Wayne, Merrill and Jay.
"I just did a show in Hawaii where I honored my four original Osmond brothers... It was their sixth decade. Two of them have retired, I got two of them out of retirement," she said. "They've soldm like,100 million records. It's, like, crazy, right? I don't think they were every honored. My brother, Merrill, I mean, he still has, like, this awesome voice. He's a rocker and he still sings fantastic."
"I mean, of course, I think Donny is average," she quipped, before adding, "Donny's very talented."
"They're all talented in their own way, but, man, Merrill's voice is just still unbelievable," she said of the 65-year-old brother.