Before I post the next chapter of TAOL... · 4:41pm Dec 29th, 2015
Because this issue has come up again from a pre-reader, I feel I must address it more completely here.
Princess Celestia and Princess Luna had parents who gave them powerful magical weapons before they left. These weapons were imbued with the powers they (their parents) had, namely, the power of the sun and the moon. Or Solar and Lunar magic, if you prefer.
Point 1.> The weapons were given to the both of them.
Point 2.> The weapons can be wielded by the both of them, they are not exclusive to either. Thus in the early days when Luna was still young and relatively weak, Celestia could have some access to lunar magic without risking Luna in a battle.
Point 3.> Their parents had no more expectation of those weapons being used by one sister against another than any parent would expect who gives or wills their firearm collection to their children.
Point 3.> Yes, the lunar sword is super effective against solar magic and vice versa, but they are no more lethal that a basic edged weapon in the short term. (I do hold that most magical weapons inhibit healing, causing wounds that have a tendency to resist blood clotting and wound closing.)
I hope I covered all issues here. Thanks for reading!