Please, before you will hate me, read my thoughts to the end^^
 
So: Actually my first idea was a pretty deep, and I know that this theme is not popular in our community but despite this fact I did a risk to think about parents of the Princesses.  
“Each segment has a beginning” and although Sisters are immortal i am pretty sure they was born by… classical way^^  
Is this so or not one I can say being sure: they survived much more than one generation.  
Which means they can remember ponies who was near to them in past and not exist at the current time.  
The places where they lived turned into ruins and thickets of dense forest. But the magic remembers all.
 
In this picture I wanted to draw a place which means a lot to sisters and a shadows of that ponies, who also brings them a lot of dear memories.  
Who they are? I thought about Princesses’s parents, but they can be anyone.
 
P.s.  
Also this picture is a test of a comic style. I thought that drawing background with sharp lines will allow to draw it faster.  
I was mistaken X)