Berthe Harstad

I am seeking information, and have not had much luck, about my 2nd great-grandfather's first wife, said to have been named Berthe Harstad. Records indicate she was born 29 Mar 1866, and that her parents were Olaf and Anna Harstad. She supposedly left Norway in 1895, bound for North Dakota. I do know that Berthe had a sister named Oline, who was born 24 Feb 1874, and I located her birth record in the parish of Stange/Tangen in Hedmark County. On later records Oline indicated her maiden name was Haroldstad. She was married to Even Trosvik and had three children before leaving Norway for the U.S.A in 1906. I am interested in finding a birth record of Berthe, as well as any information I can find about her parents, siblings, etc... and the information about her migration to the United States. Any help would be very much appreciated. Thanks!