He is married to Janet Thomson.
They got married on August 16, 1875 at Beith, Ayrshire, Scotland .Sources 1, 2
https://pigott-gorrie.blogspot.com/2017/11/robert-young-father-and-son-of-ayrshire.html
Record for William Miller/ Ancestry.com
Name: William Miller
Gender: Male
Marriage Date: 16 Aug 1875
Marriage Place: Beith, Ayr, Scotland
Spouse: Janet Thomson
FHL Film Number: 6035516