Marion McLane SHAW and Edwin Clark McGACHEY marry in 1877, and there are two children born to this marriage.
By 1881 Marion is living with John MORRISON, and this union continues until the death of John MORRISON, 1937, and the MORRISON-SHAW children can be seen in the Census.
Using FreeBDM I am not seeing a John MORRISON marrying Marion, or variation of the name, between the births of Eveline and Barbra and I suspect that there is no marriage to find.
I wonder what happened to Edwin?
The Census describes families, and the MORRISON household includes, and names, as MORRISON, the two children born to Edwin McCACHEY.
There are no name changes or deception. This was a common way to name families in the Census, a social description of a blended family.
At marriage, Eveline uses her own father's name. Brother William probably did as well. A marriage certificate was one of the few occasions when people recorded information about themselves.
It might be interesting to see how Eveline records her name on certificates as mother of her children? MORRISON or McGACHEY?
The death certificates show an attempt to include all the names that Marion used, but might not be understood by the informant.