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Celestia and Luna knew they will live forever. They are going to meet, know, befriend and fell in love with ponies, who are not going to stay by their side forever. They promised one other, they should keep a certain level of emotional distant from others, for their own sake. Among that, having foals of their own, fearing the pain of seeing their own foals grow and die before them.
So Celestia was shocked to see Luna holding a foal in her arms.
It was early morning when she was called to her sister’s room by Kibitz and Raven. They said it may be urgent. Celestia was confused, she was sure Luna didn’t return from the bat ponies yet. She was wrong, and she didn’t return alone.
Luna told her what happened, and before Celestia even could bring it up, she mentioned the promise. Even after so long she still remembered. She apologized for breaking it, but for Celestia the promise wasn’t important, she doesn’t want her sister to get hurt.
So she sat down and told her; she will help and support her no matter what. But also told her:
“I just don’t want you to regret it.”
Luna assured her she will not. And this is a promise she really keeps.