Not having any enjoy finding this couple on passenger lists to USA or Canada so far.
Have you looked at this scenerio
There is a death of a Maria Sinclair - Islington London - registered December 1898 - 1b 220
Maria was 52 years old.
Noticed a marriage for a William Sinclair to Elizabeth Adelaide Sims in Islington and registered December 1900 - 1b 699
This couple appear on the 1901 census 5 St Peter Street Islington London. Occupation would differ from the 1890 Scotland census for your William Sinclair.
William Sinclair 45 born Scotland. Insurance Agent
Elizabeth Adelaide Sinclair 39
Henry Fred Sims 7
Ethel Mary Sims 5
https://www.familysearch.org/ark:/61903/1:1:X9ZR-X4R1911 free index 155 Packington Street, Islington (interesting middle name for his son born 1903)
William Sinclair 55 born Orkney Scotland. Agent and Collector
Elizabeth Adelaide Sinclair 49
Henry Frederick Sims 17 step-son
Ethel Mary Sims 15 step-son
William Robert Sinclair 8 son
https://www.familysearch.org/ark:/61903/1:1:XWGM-JY7Birth Redg William Robert Sinclair -
https://www.familysearch.org/ark:/61903/1:1:2F82-SJ7The son William Robert Sinclair - born 1903 died January 1967 - aged 64 years - St Pancras London - 5d 986
Burial Camden
https://www.deceasedonline.com/servlet/GSDOSearch?DetsView=Summary&src=ext&fileid=1869635Food for thought anyway
Sandra