In the sister relationship, would Luna be Anna, Elsa, or a combination of the two? Also, does Luna become Nightmare Moon in this version?
 
I think:  
Luna, the younger sister, has amazing powers of moon/darkness magic. While playing, Luna would accidentally eclipse the sun and/or darken Celestia’s heart but their parents take them to the tree of harmony and Celestia is cured by the elements of harmony. The parents have Luna try to withhold her magic.  
Years pass.  
Celestia becomes Queen. Celestia is smitten by Discord. Luna slips up and eclipses the sun and/or darkens Celestia’s heart. Luna sings Let it Go. Celestia chases after Luna and leaves Discord in charge.  
I’m not quite sure where Luna goes to… the Moon, the castle of the ancient sisters, something???  
etc…  
||Discord tries to kill Queen Celestia and inherit the throne and convinces people they married, blaming the death on Luna. Luna protects Celestia from Discord. Celestia dies of darkened heart / eclipsing of the sun. Luna’s act of love for Celestia undarkens Celestia’s heart. Luna un-eclipses the land. Happy end. ||

Cute.

“Tia, want to build a snowman?”