I found an obituary for Petra! From the "Garfield Enterprise," 19 October 1900:

Mrs. Petra Olson, wife of Martin Olson, was born in Denmark Sept. 27, 1868 and died at her home in Garfield, Wash., Oct. 15, 1900, aged 32 years and 18 days. She was a member in full fellowship and good standing of the Lutheran church. A devoted wife, an affectionate mother, and a kind neighbor. Her life was most genial, generous and pure. And all who knew her gave testimony that Mrs. Olson was a good woman. She leaves a father and mother, two brothers and one sister, a broken hearted husband and six little children to fight the battles of life. May God's grace be sufficient for them in their afflictions. In the city cemetery in Garfield sympathizing friends laid all that was mortal of sister Olson to await the resurrection of the dead. Funeral services were conducted by the writer. Signed Geo. F. Stivers.