Sheryl Blanksby, from Australia, noticed all had gone quiet in the room where she had briefly left her two sons, three-year-old William and four-month-old Thomas.
So she decided to sneak in with her camera to capture their antics, but she wasn’t prepared for the emotional scene that greeted her.
“I was expecting my baby to be covered in Nutella or something, but instead I saw this,” she wrote on Instagram on Tuesday 7 March.
“My preschooler is telling his baby brother ‘Kuya [Filipino for big brother] is here. Everything is ok.’”
Thomas has been diagnosed with a rare and aggressive form of cancer called a malignant rhabdoid tumour over his kidney and in skin lesions.
Tumours have also been found in both of his lungs, on his hip bone and his right femur bone.
Sheryl and her husband Jon told The Daily Mail Australia, they have have not been given a time frame for how long Thomas will live, but there is nothing doctors can do to save his life, all they can do is make him as comfortable as possible.
The parents have told William that his new brother has an “ouchie”.
“He is aware that mummy and daddy are very sad because Thomas is not well,” they said.
“William also said that maybe the doctors can’t help his baby brother, that’s why mummy and daddy are sad. So we think he knows in his own way.”
The photo above isn’t the only time when Sheryl has found William comforting his baby brother.
But of course it’s not all cuddles - brotherly love expresses itself in all sorts of ways.
Commenters on Instagram were touched by William’s caring nature and on Thursday 9 March, Blanksby shared a photo of her eldest son in a Superman cape and praised his big heart.
“He is indeed Super Kuya to our #littlewarrior Thomas,” she wrote.
“He is really a loving soul and I am so damn proud of him for the boy he is becoming.
“He’s a normal Threenager, strong willed and defiant, but he’s got the biggest heart and funniest laugh (not as funny as mine) and oh he sure got jokes to make everyone happy.”