Adalynn 'Addy' Sooter, from Rodgers, Arkansas, was diagnosed with an inoperable brain tumor known as Diffuse Intrinsic Pontine Glioma (DIPG) in November 2016 after her parents noticed she was having some difficulty walking.
She underwent 33 bouts of radiation that initially killed the cancerous cells, but they returned.
The family then tried their luck with an experimental chemotherapy treatment in Mexico, spending more than $200,000 for transportation and medical expenses in Monterrey - but the tumor continued to spread.
In the photo, Sooter's son, six-year-old Jackson, is seen stroking his sister's head and giving her his other hand to hold.
'A little boy should not have to say goodbye to his partner in crime, his playmate, his best friend, his little sister,' Sooter wrote on Facebook. 'This isn't how it's supposed to be. But this is the broken world we live in.'
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He will wipe every tear from their eyes. There will be no more death or mourning or crying or pain, for the old order of things has passed away. (Rev 21:4, NIV)