Nik Wallenda worries for his sister ahead of latest stunt

As Nik Wallenda prepares to walk a wire 25 stories above New York's Times Square, he admits he's a little uneasy.

And for good reason: His sister, Lijana, will join him for the first time since a near-fatal accident.

Two years ago during a rehearsal for a stunt, Lijana Wallenda fell 30 feet and suffered severe injuries to her face that required reconstructive surgery.

"Most significantly she broke her heal and as a wire walker that is extremely painful. She has a lot to overcome both mentally and physically," said Wallenda.

This time, the siblings will start from opposite ends of the 1,300-foot wire, which will be suspended between Times Square One and Times Square Two.

"The distractions of Times Square are like nothing I've ever faced," said Wallenda.

In the middle, Lijana Wallenda will sit on the wire and let her brother step over her. Both will then continue to the opposite side.

"My sister will be walking down at about four degrees and then up at about 11 degrees. It does create a monumental challenge," said Wallenda.

The attempt will air live Sunday at 8 p.m. ET on ABC.