Luna stood for a moment, hesitantly, then steeled herself and knocked on the great oaken door. Hearing nothing in response and only the faintest sound of crying from within, she knocked a second time, somewhat harder. The only response to this was the illumination of the door with a magical aura and the door opening just barely wide enough to admit her. Frowning at the indignity of squeezing through the door, the princess of the night nevertheless folded her wings completely and squeezed through the space. A moment later the door closed behind her and the aura faded away. The guards outside behaved as usual, but favored each with the slightest hint of hope. Hope that the Princess would return, it was never a good day when she barricaded herself within her rooms. Celestia's moods, and now Luna's since her return, often set the castle staff on edge and their very presence or absence plus their attitude on a given day tended to comfort or stress out the staff with some regularity.
Celestia lay on her bed, curled up beneath her wings, hiding from the world. She avoided even the faintest glimpse of the sun. Between sobs, she silently cursed herself for every little detail of this whole circumstance since that fateful day 15 years past. Why had she broken one ancient promise, kept for years? What good could it do, and to make matters worse it had ultimately been for her, and benefited her lover little. If that wasn't bad enough, she had made her daughter her student and passed on many difficult things to her, knowing that failure to complete the task Celestia set would alone have been crushing. All issues aside, she could have kept Twilight at her side had she suffered the difficulties of her existence. Perhaps it was for the better, since she would have few friends at court, but still she could not benefit from the position that ought rightfully to have been hers.
In the midst of all this, she cursed even at the intrusion of her Sister into the matter. The only pony in the last five centuries or even the last millennium who could crack her outer layer of composure just by being herself. She could keep up this mask forever, in some sense, in front of ponies whose comparatively short lives left them blind to some of the greater sorrows and frustrations of the exceptionally long lived. On the other hand, against her own mind and Luna, she had little defense. Things cracked and broke inwardly and Luna was the last straw for the outside, at least for a little while. Oblivious to the world, she wept for her folly, and her indifference, crying over the sorrows of being a pony and the inability to wholly resist emotions indefinitely.
From the corner of the room, Luna watched on - profound curiosity mixed with concern. Ever since her return, her Sister had proven to be very much a distant pony, excepting occasional moments of affection. A thousand years of separation had changed her, perhaps not entirely for the best. She had not, though, expected anything like this. For several moments, she looked on, hoping for some further acknowledgment of her presence. At last, convinced of the need to act directly, she moved slowly and deliberately to Celestia's side. She glanced back for a moment, assuring herself that the door was locked. Privacy assured, she prodded the nearest of Celestia's wings gently. Encouraged by the slightest upward movement, she crawled onto the bed next to Celestia, and sat on the bed beside her, under the raised wing.
Luna's presence brought Celestia back to reality, comforting though it was.
"Sister,.. I..", Celestia, the almighty, broke down into a brief shower of tears.
"Yes, Celly?", Luna prompted, tilting her ears and looking her Sister in the face.
"I have a dilemma... and I know what I ought to do, but I fear it will not end well..."
The princess of the night wondered silently what manner of dilemma this might be. Surely it could not be that dire or so simple as a conflict, or it would have been dealt with swiftly. What could be causing Celestia so much distress?
"If you cannot tell me what pains you so, how could I possibly help you?"
Celestia sighed deeply, a look of deep sorrow etched onto her face.
"You know that you are Cadence's great, great, however many times grandmare, yes?"
Luna tapped her chin thoughtfully, recalling what she had learned since her return and trying to make it mesh with the events of a thousand years past.
"I suppose, but I shall have to bear with you on this one as well. After all, I was not here for her birth or that of most of her ancestors."
Celestia sighed again, how often she wished she had not lost Luna to the Nightmare, it was hard to have that brought to mind every time the slightest of modern things required an explanation.
"Well, you see, Luna, the squabbles among the ponies who could claim descent from use came to head two centuries after you left. After having to explain to yet another generation of over entitled ponies that their ancestry did not make them my heirs I resolved to end their disputing... I vowed never to have any kind of personal relationship extending beyond friendship that I might never again have descendants related closely enough to have any kind of better claim than a connection twenty or thirty generations past." This information caught Luna by surprise, though it surprised her less than the things of modernity. "Of all ponies, I would not have thought you could manage that.", Luna remarked, suppressing, with some difficulty, a fit of giggles. Celestia grimaced, her Sister had not had to deal with the impudent and occasionally violent uprisings from several quarters that had attempted to depose her. Those had been terrible times for her ponies, and left her frustrated and wracked with grief at having occasionally had to put member of her family, distantly related to be sure, to death for treason. She could not be impartial, intentionally causing bodily harm and killing ponies to get to her could not go unpunished. Truthfully, it had not been easy restraining herself in such a fashion. "A century earlier I might not have, " she began, "but the hurt I suffered and the injury and death of ponies near and dear to me made it clear that it could not be. However, I fear it may yet be again..." Celestia leaned into her sister and nuzzled her, then she spoke. "About fifteen years ago, after seven and a half centuries, the latter two or three mostly peaceful, I caved. A certain pony had convinced me of his love, and I liked him as well. Nevertheless, it was a star-crossed thing. I knew it the moment it began, but I had hoped it would not end as it did."