Looking into each other's eyes as they lie side-by-side on the living room sofa, this is a photograph of a tender moment between two brothers.

But it also captures something much more poignant.

For at the time 3-year-old William was also doing his best to comfort his baby brother Thomas, who is battling terminal cancer at just 4 months old.

The heart-wrenching picture was captured by their mother, Sheryl Blanksby, who walked into the room to hear her eldest son telling his brother, "Big brother is here, everything is okay," as he gently rested a hand on the baby's cheek.

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Blanksy, from Western Australia, shared the photo on Instagram with a caption explaining how she had expected to walk into the room to find her sons getting into trouble. Instead she was greeted with this precious scene.

It is one of a number of touching online updates shared by Blanksy and her husband Jon as they fight to make the most of every moment they have with their baby boy.

Thomas, now 13 weeks old, was born with a skin lesion on his right arm that looked like a birthmark.

But after tests revealed a lump on his abdomen, Thomas was diagnosed with a malignant rhabdoid tumour and his family don't know how long he has left.

"There are tears and that immense pain of losing someone you love so much. We film him constantly and stare at him and kiss him," the couple told Daily Mail Australia last month.

"We as a family are trying to cope the best way we can, but it has been very hard."

Other than the mark on his arm, Thomas, who was born in November, showed no symptoms and was in no pain. He was a happy newborn baby.

But when he turned 6 weeks old, a doctor found a lump on Thomas' abdomen, which an ultrasound would reveal was a large tumour over his kidney.

Doctors removed Thomas' left kidney and discovered he had a malignant rhabdoid tumour.

The rare and aggressive form of cancer most commonly occurs in infants and toddlers, with only around 25 cases every year diagnosed in the US.

Rhabdoid tumours commonly first appear in the kidneys but can also originate in the soft tissues of the brain, before spreading to other parts of the body.

With a survival rate below 30 per cent no matter where the tumour originates, there is no documentation of a successful treatment.

Thomas' family said doctors then discovered that his skin lesions were also rhabdoid tumours.

Tumours were also found in both of his lungs. An additional one on his hip bone also fractured his right femur bone.

The cancer had spread. There was nothing else the doctors could do.

Sheryl and Jon have told their 3-year-old son William that his new brother has an 'ouchie' that doctors are trying to help.

"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."

Sheryl and Jon have not been given a time frame for how long Thomas will live, and can only work to make him as comfortable as possible.

They are now trying to make the most of the time they have left with Thomas, and hope to give him a normal of a life.

"We want to travel with the boys, go on weekend trips to the beach, picnic at the park, drive down to Margaret River or go further down to Albany," they said.

"Go on fruit picking trips, outdoor movies, family photo shoots, water parks, or even a hot air balloon if that's allowed for a three-month-old."

The couple's friends are raising money through a GoFundMe campaign so they can enjoy the time they have left with Thomas without the stress of hospital bills.

Although they will lose him soon, Sheryl and Jon said Thomas is teaching them that life is what's truly important.

"We often get caught up in our own little world of first problems and we sweat the small stuff, or complain about things that don't matter," they said.

"We forget how to be grateful of the things and life we have. We must live with a purpose, gratitude, and magnitude of love."