Donny and Marie Osmond took the stage in Las Vegas together last night for the first time in 35 years, and ET's Thea Andrews was front, back and center for all the Osmond mania!

An hour before the curtain rose on the opening night of their show, the brother and sister duo dished on their last minute preparations, how they planned on celebrating afterward -- and whether they felt nervous or not!

"Marie's gonna try to show me up -- ain't gonna happen baby," Donny teased.

Their show runs through July 23 at the MGM Grand.