Si(y)dney Knox and Robert Knox

Am looking for ancestors of Si(y)dney Knox (yes, Knox is her maiden name as well as her married name) and Robert Knox, my great-great grandparents.  Here is all the information I have:

James McIntosh Matthews, born 19 October 1857 in Almonte, Ontario, Canada, in 1900 resided in Farmington, Herman & Leroy Townships, Lake, South Dakota, and died 12 May 1913 in Madison, Grant, South Dakota.  In 1880 he immigrated to the US from Canada and in January 1888 married Sidney Knox at the home of Sidney’s uncle in Preston, Minnesota.   Both James and his wife Sidney are buried in Graceland Cemetery in Madison, Lake County, South Dakota. 

Sidney Knox was born 5 June 1860 in Irvinestown, Fermanagh County, Northern Ireland, came to the US in 1875 and died 3 October 1931 in Omaha, Nebraska while visiting her daughter Laura Mabel Kopperud and her family. Sidney’s parents were Robert and Sidney Elizabeth Knox.  In the 1900 US Federal Census Sidney said they were both from Ireland.  In the 1920 Census, Sidney said her father was from Scotland and her mother from Ireland.  In the 1930 Census, she said that both parents were from Scotland.  A birth certificate has been found for a Mary Jane Knox, parents Robert and Sidney Knox, born in Fermanagh, Ireland 8 November 1864.  This possibly could be Sidney's sister. 

James and Sidney’s children were Irvin James (1888 - 1972), Laura Mabel (1890 – 1985), Joseph George (1893 - 1963) and Della Irene (1901 - 1984). 

Records found on Robert and Sidney Elizabeth (Sidney Matthews referred to her mother as Elizabeth but records indicate that she was Sidney so perhaps Elizabeth was her middle name) Knox include their civil marriage on June 1, 1852 Parish Magheraculmoney, Civil District Irvinestown, County Fermanagh.  Born in 1833, father John Knox, Sidney Elizabeth died on 24 March 1900 and her will left the administration of the estate as follows:  “Sidney Knox late of Shanmullagh County Fermanagh who died 24 March 1900 granted at Armagh to Robert Knox Farmer the Husband, effects £50 18s. 9d.”

Robert Knox, father William Knox, was born in 1827 and died between 1901 and 1911.   Some additional information on the family:

1901 Irish Census
Shanmullagh, Co. Fermanagh

Knox, Robert, aged 80, Head, religion given as "I.C.", born Co. Fermanagh
Knox, Joseph, Son, aged 25, married
Knox, Margaret Jane, Daughter-in-law, aged 19, married
Barrett, Mary Catherine, Servant, aged 20, Roman Catholic, born Co. Tyrone

1911 Census
Shanmullagh, Co. Fermanagh

Knox, Joseph, 42, Head, Church of Ireland
Knox, Margaret Jane, 29, Wife, Church of Ireland
Knox, Robert, 9, Son
Knox, Sydney, 7, Daughter
Knox, Mary Jane, 5, Daughter
Knox, William John, 4, Son
Knox, George Herbert, 2, Son
Knox, Edith Olive, 6 months, Daughter

Any help on more information would be much appreciated.