Nik Wallenda calls sister’s high-wire stunt after major accident 'almost impossible'

Acrobat siblings Nik and Lijana Wallenda, along with a crew of workers, are making final preparations for an epic high-wire stunt in the middle of Times Square.

On June 23, the daring duo will walk one of their most challenging routines to date between two New York City skyscrapers 25 stories (more than 1,000 feet) in the air.

The siblings will start on opposite ends between the famed 1 Times Square and 2 Times Square. The most harrowing moment will be when they cross each other in the middle of the wire with live crowds watching from below.

“It’s a little intimidating,” Wallenda admitted on "Good Morning America."

"One of the challenges that we have is we’re at 17 stories here, but we're at 25 stories there," he said, pointing to the end of the wire.

He continued, "It's pretty steep on that end, a little more than I expected ... so it's stressful for me. I have to walk down that incline so it's extremely intimidating for me just standing up here looking at it."

Both will be wearing a safety harness that will be tethered to a wire the entire time, even when they cross paths, which was required by the city and ABC, which is broadcasting the spectacle.

ABC News has learned that the production crew has experienced problems with the rigging process.

"It's always changing -- it makes it a nightmare -- so you never know what's going to happen," Nik Wallenda said.

The executive producer of Sunday night's show told ABC News the race to install the wire rig "proved more challenging than Nik ever imagined" and that his "team will be making adjustments to it all week because there is zero room for error."

The two-hour televised event marks Lijana Wallenda's first high-wire walk since an accident in 2017 when she, along with four other acrobats, fell off a tightrope during a rehearsal.

"I broke a rib, punctured my right ear canal, broke clear through my left humerus, I broke my left calcaneus," she said. "But the big one was every bone in my face."

The siblings, seventh-generation members of the Wallenda family circus troupe, have trained for the worst possible conditions. They've used wind machines with 90 mph winds and hoses blasting water to simulate torrential rain.

"I've done some big events in short periods of time, but nothing compares to this," Nik Wallenda said.

Two years since the tragic accident, Lijana Wallenda will have to confront her nerves before the physical and psychological challenge.

"I'm just a little nervous because the wire moves more than anything I've ever been on," she explained.

"There is that physical aspect that she hasn’t been on the wire for a long time, so that’s a major concern," her brother said. As for the mental aspect, "I have concerns that when she gets up here I don’t know that she’ll take that first step. I think she will, our family blood says never give up, continue on," he said.

"What she’s having to overcome to me, is almost impossible, and I’ve done this my whole life as well," he said. "This is the highest walk by double anything she’s ever attempted before and the longest by quadruple."

"Highwire Live in Times Square with Nik Wallenda" will air live on Sunday, June 23, from 8 p.m. to 10 p.m. ET on ABC.