Riis was her maiden name, according to her death certificate her father's name was Frederick B.A. Riis, her mother's name was not listed. She married a man named Chiel (anglicised to Charles intermittently later in life) Mortensen sometime in the early early 1880s. Mortensen was a Norwegian who in various census records is sometimes listed as having been born in Germany. Family folklore says that his father worked for the Norwegian government in Germany, but there is no documentation to prove this.
Fredrica (also spelled Frederica and Fredrikka) had ten children. Her first three, Esther (unknown birthdate), Anna Isabel (my grandmother), Frederick were born in Norway. Esther in an unknown place, Anna in Hammerfest in 1884 and Frederick in Trondjen (Trondheim?) in 1887. Except for Esther, they were all in the US by the 1910 census living in Westchester County, NY.
Thanks for taking the time to help.