The love of a brother or sister is like no other. Despite the arguments, sibling rivalry, and any family drama, they'll always be there for you.

Nobody knows this sentiment to be more true than six-year-old Jackson Sooter, who comforted his little sister, Adalynn, four, moments before she passed away on June 2.

Their father, Matt Sooter captured the shot, which has since went viral on Facebook.

"A little boy should not have to say goodbye to his partner in crime, his playmate, his best friend, his little sister," Sooter captioned the picture. "This isn't how it's supposed to be. But this is the broken world we live in."

"It was a sweet moment, but not unexpected," Sooter later told PEOPLE. "Jackson has always been great with her and took very good care of her. He's still a happy little boy, but he misses her."

Adalynn, or "Addy" as she was called by her family, passed away after an 18-month battle against diffuse intrinsic pontine glioma, an "aggressive and hard to treat" brain tumor.

After Addy was diagnosed in November 2016, her parents were told she only had months to live. However, she shocked hospital staff in Rogers, Arkansas when her tumor began to shrink.

But despite the good news, they didn't expect Addy to beat her cancer.

"While we're doing everything we can to change the outcome we don't expect to win this fight. We haven't given up, but it seems we are losing the war," Matt said at the time.

After dozens of failed radiation treatments, doctors told Matt and his wife Chandra there was little else they could do for their daughter. But the family wouldn't give up on Addy, and they sought experimental treatment in Monterrey, Mexico, which wasn't cheap.

According to Matt, a single trip could last 12 days, and in total cost the family $200,000.

Unfortunately, that treatment hadn't been effective either, and the family, including little Jackson, prepared for the worst.

Following Addy's death, Matt said the family is in mourning, but know they'll get through their tremendous loss together.

"We miss her terribly, but we're also at peace knowing we did everything we could to try to help her and that she isn't in pain anymore," Sooter said. "It was only, 'See you later.' We'll see her again some day."