Born in 1848 in the Red River settlement to parents Jean-Louis Riel and Julie (Lagimodière) Riel, as a young child she was surrounded by the word of God, being educated by Sisters of Charity of Montreal but also by her mother, who was dedicated to the Christian faith.
She was inspired to become a nun after her brother Louis Riel entered the seminary to become a priest.
Although Louis did not become a priest, Sara actively took her Solemn vows in 1868.
She taught languages and arts at the Grey Nuns boarding schools between 1868 and 1871, after which she became a Catholic missionary.
Although separated from her family, her writings showed a sympathetic view on the eve of the resistance.
Given that her brother was an active member in leading the revolution against the government, in 1869 the congregation feared for her safety and she was moved several times within a few years.
In 1872 Sara Riel was taken ill and almost died, after a vision from God, Sara Riel changed her name to ‘Sister Marguerite-Marie of Alacoque’ and re-honored her vows and commitment to the Catholic Church.
Sara actively wrote to her family telling them of her experiences, wishes, and hopes for them.
She died of tuberculosis in 1884 at the age of 35.