They have a very special bond.
So when eight-year-old Noah Aldrich heard about a youth triathlon happening in his local area, he immediately knew he wanted to take part with his little brother.
The pair, from Idaho in the US, began training the next day and continued to train for three months before the event, reports the New York Daily News.
But the extraordinary thing is Noah’s little brother Lucas, six, was born with lissencephaly – a rare brain disorder that means he will never walk, talk, or eat on his own.
So on July 12, the brothers took part in a 200-metre swim, three-mile bike ride and one mile run, with Noah carrying little Lucas in the sweltering heat the entire time.
For the swim, Noah pulled Lucas along in a raft before his little brother was transferred into a bike trailer for the cycle and a buggy for the run.
“It was a little nerve wracking but exciting and exhilarating at the same time,” mum Alissa Aldrich told the Daily News.
“And we're definitely proud. As a parent it was a pretty incredible thing to watch.”
Despite Lucas’ condition, the family still like to remain active and they’ve all been on a flight together, been adaptive skiing and on a trip to Disney World.
The two brothers are especially close and their mum says Lucas ‘lights up’ when he sees his big brother and starts moving his arms and legs excitedly.
They love to hang out together, watching cartoons and going on walks and bike rides as Noah pushes him in his buggy.
But the triathlon is the biggest thing they’ve done together, and mum Alissa said Lucas loved the whole experience.
“I think [Lucas] definitely knew he was part of something pretty special,” she added.
"When we got him out of the water, he had the biggest grin that you would have ever seen (and) I know he enjoyed the bike and the run," she said.
"Our goal with the whole thing was just to have fun. You can live life without limits."
The family has set up Lucas’ House, a place where children in Idaho and the surrounding area can come and receive respite and end of life care.
You can find out more information on the Lucas’ House Facebook page.