Bill Klein and Jennifer Arnold with their children, Zoey and Will
02/20/2014 at 03:00 PM EST
What sibling rivalry?
Though it's been a rough journey for Dr. Jennifer Arnold and Bill Klein of TLC's hit reality show The Little Couple
after her cancer diagnosis
and subsequent treatment
, one thing they didn't have to stress about was how their son, Will
, 3, would take to his newly adopted sister, Zoey
"Will is an awesome big brother," Arnold, whose cancer is now in remission,
tells PEOPLE. "He's unbelievable. He has this amazing, happy personality, and he's shown her how to play with the iPhone, which she loves."
Since bringing Zoey home in October, Will has taken his new job as big brother very seriously, pushing his sister around on riding toys and sharing his things with her.
"We are so lucky that Will is who he is," says Klein. "I've never met a kid like that, and we're lucky that he's ours."
Arnold, 39, who is a neonatologist at Texas Children's Hospital
, and Klein, 39, a business owner who runs their Houston-based pet store Rocky & Maggie's Petshop
, have a form of skeletal dysplasia, as do both Will and Zoey.
"I can hope the world is a slightly better place than it was for us," Arnold says, "and that it is better for them."
The new season of TLC's The Little Couple
premieres Tuesday, March 4, at 10 p.m.
For much more on Arnold and Klein's past few months, including more exclusive photos of Zoey, pick up this week's issue of PEOPLE, on newsstands Friday.