Looking for Sander Syverson family

I am trying to locate where my great great grandparents came from in Norway and any other family members. The following is the information I have: Sander Syverson born about 1813 by US census records. Birth year varies between census from 1810 to 1815. Sander is used consistently but Syverson is spelled several different ways Ragnild Syverson born about 1817 varies between 1815 and 1825 but census data shows about 5 years difference between Sander and Ragnild in age. Ragnild is never spelled the same in any census. They had three children born in Norway daughter Christina born about 1849 daughter Julia born about 1851 or 1852 son Syver born about 1855 varies from 1853 to 1856, sometimes spelled Sever or Siner Sander's Naturalization records give June 15 1860 as the date of arrival in America. They homesteaded on section 2, New Sweden township #111, range 28 in Nicollet county, Minnesota a few miles out of St Peter, Minnesota. Norseland was their closest town for post office and church. If they continued to use Norwegian naming systems the following three children born in America may give clues to family members in Norway son Sander Alexander born about 1861 daughter Lena or Dina born about 1865 son Peter born Dec 28 1867 my great grandfather. Also Peter and Syver both named their first son Selmer. This may not have any relation to relatives in Norway as they were born around 1890. All of Sander Syversons children used the surname Sanderson. These names have all been Americanized. I have not had any success searching Norwegian records as I am not sure of the most probable Norwegian spellings. Any help would be much appreciated.