Was being in charge of the sun and moon really the destiny for both Celestia and Luna or did they just do it because they claimed they were more powerful and could easily do it compared to the unicorn wizards that did it originally? Something tells me that they just decided on their own to do those. Usually when a pony gets their cutie mark, they usually get it by accident with something that they do out of the ordinary for themselves. Then again, maybe it was their destiny to be in charge of those. I only say this because it has been on my mind for some time, and I wanted to know if it really was their destiny to control those or not especially when the Journal of the Two Sisters doesn't make it very clear on that.

I would say this is in the realm of headcanon, and any writer of fanfic can make up their own scenario for this.

When I did Royal Origins, I had them earn their cutie marks by accident just like any other pony, though that was before they were alicorns, which I still think they weren't born as.

If the generations were related, then Megan really did meet the Royal Sisters when they were foals.