Yes, Mary Ann was Henry Canaday's stepdaughter. Mary Ann's elder sister Sarah Jane was my ggg-grandmother. They had another sister, Ellen Amanda, and a brother, Columbus Webster Prouty.
Mary Ann's mother, also named Mary Ann (nee, Wilson), married Henry Canaday in 1849 in Baltimore, Iowa after the death of her first husband, Austin Prouty. As you probably know, the family came west to Oregon in the summer of 1852.
I am just beginning to work on Sarah Jane's siblings in the hopes of learning more about her. She and her husband lived in Myrtle Creek for some time before moving to Washington, hence the reason I am looking for clues from Mary Ann's obituary if one exists.