Alongside the poignant picture, her mum Ally Parker confessed that the family were in a "terrible place" due to the diagnoses.
Braylynn was diagnosed with a brain tumour in December, while Sean suffers from motor neurone disease and no longer has the ability to speak.
There is now no effective treatment for DIPG, which is extremely resistant to chemotherapy and experts say has a 0 per cent survival rate.
"In a few days I will have to bury this attractive little girl", the mother posted on Facebook.
A USA woman has shared a heartbreaking picture of her terminally-ill daughter and father together after being told they will likely die within weeks of each other.
But Parker has since received what she is calling a "glimmer of hope". "Months, maybe even weeks, later, I will have to bury my father", Parker lamented.
"Both of my heroes, gone, within the same year.How could this happen to us?" Why do they have to leave us?
"This world is a awful place".
Close Braylynn Lawhon with her grandfather Sean Peterson
The Florida family has set up a Facebook page and a GoFundMe campaign in order to raise $300,000 for an experimental procedure in Mexico.
The treatment, not covered by insurance, cost almost $30,000 a round, and due to the aggressive nature of this cancer, several rounds are needed for success, The Advertiser reported.
'My baby girl deserves to live a full, happy life, and so does every other child who has had to face DIPG, ' Parker writes on the social media site.
She added: "Last year was hard for us, but I can't even begin to explain how difficult this year will be and has already been". Today will be the worst day of my life so far, and in less than a week, I will have a day that is even more awful. Peterson was reportedly in the room to say goodbye to the girl.
'We have to put an end to this.
"I couldn't be more proud of my Braylynn, she's always been a tough kid but I severely underestimated her strength". We HAVE to find a cure, not a damn band aid.
Though the mother is fighting a battle herself seeing her daughter in such pain, a doctor who believes the little girl still has a chance to live has gotten in touch with Parker.