There were three additional children on the 1891 census that I didn't catch the first time I looked at it. Robert is at the end of a page. At the top of the next page are William G., age 9, Daniel A., age 7 and Emma E., age 4.
In the 1901 census Robert BURR Sr. is still in Bothwell, Ontario. He is remarried to Mary, age 38. Daughter Emma, age 14, is still at home. He has another daughter with the new wife, Sarah, age 1. Also two stepchildren, Ronald and Ethal CAMPBELL
The more information you can provide, the easier it is to help. Names of parents, spouses, children and a time frame in which the person lived.
I did a quick search and found a post you made elsewhere back in 2001. With the info in that post I was able to find the following:
A MT marriage record for Robert L. BURR, age 49, b. Thomasville, Canada, to Margaret PAYNE, age 34, b. Eden Valley, MN, on 15-May-1927 in Cut Bank, Glacier Co, MT. Groom's parents were Robert BURR and Nancy GRONT. Bride's father was John MCGOWAN. Both parties had been previously married.
A possible match in the 1891 census for Bothwell, Ontario, Canada:
BURR, Robert age 48 b. England
dau Elizabeth age 21 b. Ont.
dau Mary M. age 19 b. Ont.
son John C. age 16 b. Ont.
dau Martha L. age 14 b. Ont.
son Robert L. age 12 b. Ont.
I can't seem to find the family in the 1881 census, but they were in Bothwell in 1871. Robert, 28, b. Eng, wife Nancy L., 31, b. Ontario, dau Elizabeth L., age 1, b. Ontario.
Did Robert and Margaret stay married? There's a Robert L. BURR, age 51, b. Canada, father b. England, mother b. Canada, in the 1930 census in Shelby, Toole Co, MT. He is listed as divorced and is living as a boarder in the household of Max BRODERSON. He gives his year of immigration as 1922.
In the 1900 census for Cando, Towner Co, ND there's a Robert L. BURR, age 24, b. Canada in June 1876, father b. England, mother b. Canada boarding in the household of Furber KILBY. This time he says he immigrated in 1895. Also boarding there is John C. BURR, age 28, b. Canada in Aug 1872, father b. England, mother b. Canada, immigrated 1891. This is presumably the same John C. listed in the 1891 census.
You mentioned that your father had an older half sister named Clarissa. The 1911 and 1916 Canada censuses for Alberta have a Robert BURR b. June 1878 in Ontario with a wife named Mabel, and a dau named Clarice who was born somewhere in the U.S. in Apr 1905.
I am not related, I just saw your post and thought I would take a look around to see if I could find anything. I think Thomasville might actually be Thamesville, which is located near Bothwell. Both are a bit northeast of the Chatham-Kent area of Ontario.
I think it is safe to say that if he named Robert BURR and Nancy GRONT as his parents on his marriage license that those were their names. Though perhaps Nancy was a GRANT and not a GRONT. Finding a birth record might be difficult. According to what I've read, Ontario didn't pass a vital records law until 1869 and didn't enforce it until the 1880s. If your grandfather was born 1876-1878 he might not have been registered. According to Ontario's GenWeb page, records for 1869-1913 are available at Ancestry. I do not have a subscription there, so can't look for them for you.
I just stumbled across a Lewin BURR b. 1819 in England, living in Zone, Bothwell, Ontario in 1871, page 3, house 12 on the census. Robert Sr. was also in Zone, Bothwell in 1871, page 10, house 35. I think it very likely that Robert is either Lewin's son or some other close relative. The site that I usually use to look at census images is down right now, but I can check tomorrow to see who else was in Lewin's household.
This Lewin BURR was in Euphemia, Bothwell, Ontario in 1881 with his wife Mary, who was also born in England.
A Lewin BURR died 26-Sept-1886 in Zone, Kent, Ontario.
It's a fairly unusual name. I found a Lewin BURR b. 5-May-1818 in Folkingham, Lincoln, England, son of John and Ann BURR.
Here is a link to the tree you may have seen early that show's Ann's maiden name as LEWIN. Unfortunately it stops at Lewin BURR and does not show his children or grandchildren: