The Two Sisters were not born alicorns. In fact, until Flurry Heart, there were no naturally born alicorns. Under mysterious circumstances, these two ascended to the regal forms seen today. But before that, they were children, raised by the unicorns. Well, namely by one unicorn: a simple librarian named Stygian (who would later be known as the great Starswirl the Bearded).
Stygian was tasked with raising the foals to become the greatest rulers Equestria had ever seen. It just so happened that he was an expert on the Elements of Harmony — objects the girls were slated to use to fight against the Windigo menace — so it was up to this humble, yet nonetheless grumpy, unicorn to change the fate of Equestria for good. He was to teach them the ways of magic and all things good in the world. They would be taught to be noble, intelligent, yet caring leaders. In short, they would be his students. No more.
But Celestia-be-damned did those little tykes grow on him. They were feisty girls, yet nonetheless eager to learn everything Stygian could teach them. They regarded him as their father, and it wouldn't take long before Stygian took to his newfound family. One he never knew he wanted. It was because of his daughters that he himself was able to ascend to the legend everypony knows him as today.
-born a pegasus during the Summer Solstice; deeply connected with the sun and summer fun
-has many mysterious powers, including clairvoyance
-worshipped by her subjects nearly to a godlike level; most ponies do revere her as a deity and put a lot of faith in her. She is also the epitome of beauty, a pony everypony strives to be.
-No one knows how old she is, but she hardly looks a day over twenty. Despite the numerous battles she has been in, she does not scar. In fact, any wounds she obtains, no matter how grievous, disappear within a day, faster if she's in sunlight.
-She is very guarded with her emotions, and only expresses herself around those she's closest to, namely Twilight Sparkle and Luna. Most ponies see her as a calming, reverent goddess capable of unfathomable magic.
-As Stygian before her, she views Twilight Sparkle and Sunset Shimmer as her adopted daughters; albeit in a heartbreakingly detached way that Celestia has become accustomed to over the years, for she knows she will outlive them too.
-Celestia fell in love with King Sombra before his heart turned to darkness
-Feels a terrible guilt for banishing her sister to the moon
-born a unicorn during the Winter Solstice under an extremely rare full moon; naturally connected with the moon, nighttime and winter
-Can walk in ponies' dreams, offering comfort and guidance where needed
-A much more vocal and energetic princess, almost childish, compared to her older sister. She is loud and does not understand social graces, namely due to being trapped on the moon for 1,000 years. She is aggressive, eager to fight any challenge that comes her way, and openly passionate about the things she cares about. Somewhat difficult when her sister is comparatively emotionless.
-Before transforming into Nightmare Moon, Luna was consumed by fear over her place in Equestria. This occurred not long after Stygian passed away, leaving Luna in an existential nightmare. It didn't help that Celestia had become reserved, leaving Luna feeling isolated from all sides. Her sadness attracted the umbrum of the North, taking hold of her and turning her into the villain whose defeat would unite the greatest heroes of Equestria. It was only then that Celestia had realized she hadn't been there for Luna, and she performed the imprisonment spell at great cost to the both of them.
-Speaking of those 1,000 years, Luna has since come to terms with her mistakes. She works extremely hard, harder than Celestia, to make up for her transgressions for she undoutedly feels a deep sorrow for betraying her sister and her subjects. It seems only fair that she now spend the next 1,000 years being the princess and sister Celestia needed all along. (I don't like that Luna would summon a damn dream demon to punish herself that would further endanger her subjects, so uh, yeah fuck all that noise)
-As Nightmare Moon, she transformed a following of ponies into bat ponies. A population of these ponies still lives in Hollow Shades and comprise some of the richest, most well documented blood-lines in Equestria. Luna is not sure how to feel toward these ponies with their snobbish and frankly unfriendly demeanors, but she treats them with utmost respect and care. After all, it's her fault they exist at all.
-After Luna returned to Equestria, things were, to put it lightly, very rocky between Luna and Celestia. Luna spent a year travelling Equestria, away from Celestia, to re-acclimate into the world. She regained her strength, forged her own armor from meteorite and adapted to her new life. Their relationship is better these days, but it will never be the same as it was pre-banishment, hence why they could no longer wield the Elements of Harmony even if they wanted to.
I think that just about wraps up my ideas! I realize I hadn't updated my thoughts on them since the whole Tempest Mountain story I've since abandoned. I didn't really care for the idea of alicorns existing as natural creatures, mainly because Celestia and Luna are apparently replaceable. Unicorns, as a team, can do anything they do and their power in Equestria seems conditional. Their subjects seem to be allowed to do whatever they want basically (like, the racist Neighsayer was allowed to even exist in the first place is hella weird in a supposedly Utopian society. If he was given so much power as to dictate whether a school opens, and even Celestia couldn't override him, what exactly can she do?)
I'll delve deeper into what the princesses actually do and how they were transformed into alicorns in the comics , but they function better as transformed ponies rather than as natural beings imo. Flurry Heart and Janus are both exceptions due to exposure to unique magic — Flurry to the Crystal Heart and Janus being borne from an immortal (not a true immortal though; they have a 10,000 year lifespan) alicorn.
Let me know what you think! I love hearing folks' ideas on these two very interesting ponies!