Hi Kate
I've been looking through all the info on your family!
When Elizabeth married David Brown in 1878, aged 42 and a washerwoman, her father is stated as a Tax collector - same person as on Mary's death certificate though. With them on the 1881 census, besides Mary Ann, aged 92 are an Elizabeth Stewart, aged 15 and John Smith, aged 13, both recorded as stepchildren to David Brown.
On the 1861, Mary and Elizabeth are clearly written as McKean, have you tried searching for variations in spelling on the earlier censuses.
I'll try in my local records for Kirkcudbrightshire.
Regards
Gadget