I've made one for Celestia and I've made one for Luna, so i'm going to make one for them both and their relationship.
I've seen the fandom with the relationship between the two sisters. They range from simple sisterly love to complete and utter clop-fic debauchery. Both of which are fine, I don't judge if you are into Lunestia pairing (mainly 'cause sometimes they are just so cuuuute together but i digress). The point of this post is not their CURRENT relationship, but what their relationship between each other was during their childhood, and during their reign up until the point of Celestia having to banish her sister to the moon.
Now this is fully speculation on my part, so keep in mind this is NOT Canon in any way shape or form (Unless the writers of the show agree with me and actually make it Canon, if so /)^3^(\ )
When they were but foals Celestia and Luna were the greatest of friends and best sisters. They did what any other child would do and just explore their world to their hearts content. At some point in time they met a Tiny draconequis (however you spell that word) named Discord and this was well before Discord ever had any crazy loco problems, for years on end they had fun until something happened... but that is for another time when Discord gets his own post.
Fast forward into the future, and Celestia and Luna are new to the throne, Discord was turned to stone not even a week ago and they have just been crowned Princesses of Equestria, and things are putting a big strain upon their relationship. Celestia, being the elder of the two, has been taking on the bigger workload of paperwork and politics so Luna wouldn't have to do so much, things are stressful but not overly Complicated, as time went on things mellowed out as they solidified control over their doman that is Equestria. But things were not all well in with the citizens of Equestria when it came to the two princesses, While the epic clash between the sisters against Discord is now but a distant memory. The stories of the creatures that came out during the night has been passed down from those who survived Discord, making those who heard these stories wary of the night, and as time went on and people forgot all about Discord causing these evil creatures to come out at night, they shifted the blame slightly to Luna and her Night. So whenever she would try to see why the ponies didn't love her night she would always be met with closed doors and windows or if some unlucky pony was out at night, with the sight of a pony running away in fear of her. While in the Daytime they would be out frolicking and laughing without a care in the world. Now Luna could handle these things, she doesn't need her subjects to love her, as long as she had her Sister loving her she would be content with that... but sadly Celestia was too wrapped up in the politics and work of being Princess to properly notice her Sister, and just kept pushing her away. The final straw was when Luna made one of her best night skys yet, and when she went to show Celestia... All she did was say "its just the night sky"... and thus began Luna's descent into anger and darkness, as Nightmare Moon was born that night inside her mind.
And that, my dear little ponies.is my fanon of the relationship between Luna and Celestia, from Foalhood to the day in which Nightmare Moon was born.
I hope you enjoyed my unintentional storytime, I honestly did not mean to do so in the least, it just... kinda popped on out ^^;