Part of a two-part commission.
 
This takes place in a slightly altered history, where shortly after NMM, a second Windigo Winter swept through the land but everyone survived as a whole.
 
Denith and Alatea were two refugees, fleeing the cold winds and snow to Ponyville, where they met and chose to support each other through the dark winter.
 
Alatea is blind, and Denith has a crippled hind leg so they both rely on eachother as guidance and physical support. After a long while as sisters, they eventually fell in love. It wasnt too long until marriage was proposed
 
FYI Denith is the pegasus, Alatea is the unicorn
 
The other part: http://derpibooru.org/468292