Pope Francis will canonise Francisco Marto and his sister Jacinta Marto on 13th May when he visits the site where the reported apparition occurred.
Now one of the most famous places of Christian pilgrimage, millions of people visit the Fatima shrine in the central district of Santarém every year.
In the last step before sainthood, the siblings - who died during childhood, several years after the reported vision - were beatified by Pope John in 2000.
Their third sibling, Lucia Dos Santos, who also allegedly witnessed the vision, became a nun and died at the age of 97 in 2005.
At least two miracles are usually required for a person to become a saint.
The siblings have been connected with the healing of a Brazilian child whose parents prayed to the siblings.
Catholics also believe that Mary gave the youngsters three messages, the so-called secrets of Fatima.