In 1881 Simon McKenzie appears to be in Partick, Lanarkshire
Jessie McKenzie head, married age 33, b Lochgoilhead, Argyleshire
James son age 12, b Glasgow
Simon son age 7, b Glasgow
Maggie daughter age 0, b Partick
Jessie daughter age 0, b Partick
So if Simon McKenzie and James were brothers, is it this James who married Isabella? If so it might sort of explain Simon's presence in the 1891 census at the same residence as Bella. Bella and Jessie are shown as Cleminson although Jessie's birth seems to have been registered as McKenzie in 1889 - have you got the birth cert. to confirm the father was James?
James McKenzie & Isabella married in 1892 (freebmd) so that explains why Bella was shown as Cleminson on the 1891 census.
Shirl