It appears that he was the son of James Johnston and (Harriet?) Mastoor from the marriage and census records found. You might want to plug in the names to see if you can find more of his siblings in the 1881 Canada Census as census data indicates the family immigrated to the US in 1888. His sister Hannah/Annah is listed on his marriage record as a witness. 1900 Census notes a sister Sarah. Approximate dobs on 1900 census for all family members. Use the free search tool at: https://familysearch.org/ Examples below:
Michigan, Marriages, 1822-1995
https://familysearch.org/pal:/MM9.1.1/N324-Y1N
Groom Fred'K Wm Johnston
Age: 28 years birth date: 1882 birthplace Canada
Employment Hardware
1st marriage
Father James Mother Mastoor
Marriage date: 12 Jan 1910
Marriage place: Kinde, Huron, Michigan
Official W H Mitchell, Minister
Bride Mabel E. Hodgins
Age: 24 years birth date: 1886 birthplace: Canada
1st Marriage
Father: Elijah Mother Gibson
Witness Harriet Johnston Residence Bad Axe
Witness Maggie E Hodgkins Residence Bad Axe
United States Census, 1900 ED 6 Colfax township Bad Axe village, Huron, Michigan
https://familysearch.org/pal:/MM9.1.1/MSM6-T4F
https://familysearch.org/pal:/MM9.1.1/MSM6-T4F
head James Johnson M 56 Mar 1844 Canada Eng married 33 yrs immig 1888
wife Harriot Johnson F 55 July 1845 Canada Eng 6 children 6 living immig 1888
dau Annah Johnson F 27 Dec 1873 Canada Eng Single
dau Sarah Johnson F 22 Feb 1878 Canada Eng Single
son Fred Johnson M 19 Dec 1881 Canada Eng Single
United States Census, 1910 Dwight, Huron, Michigan
https://familysearch.org/pal:/MM9.1.1/ML58-4P1
self Frederick H Johnston M 28y CAN Immigration 1888
wife Mabelle E Johnston F 24y CAN Immigration
United States Census, 1920 Wayne, Michigan
https://familysearch.org/pal:/MM9.1.1/MZ7G-63D
self William F Johnston M 37y CAN
wife Mabel Johnston F 33y CAN
son Rex Johnston M 9y Michigan
