After Celestia learned about Luna's relationship with Twilight, she reminded her younger sister of a bet they made long ago. Considering that she is Luna's marefriend, Twilight decided to head to Canterlot and join them. Once they arrived they headed straight to Celestia's sleeping chambers where she promptly locked the door. She smacked Luna's flank with her tail and asked, "Well Lulu, shall we get back to the fun we used to have so long ago?"
Before Luna could reply Twilight asked, "About this fun, what is it? You said on my balcony that Luna liked it, I want to know exactly what it is."
Luna replied, "Twilight, to truly understand it thou wouldst have to understand the close nature of natural born Alicorn sisters."
Twilight asked, "Close nature?"
Celestia began, "Yes Twilight, close nature." She cleared her throat and continued, "Here is the thing, given that there are so few Alicorns in this world, much less natural born ones, pretty much nopony knows about the true nature of Alicorns and the bonds they forge when they are young."
Twilight asked, "So what kinds of close bonds do they forge?"
Luna replied, "Well in the case of Alicorn brothers, while they do care for one another, they fight one another to the death over territorial control. As time passed, their aggressiveness only became worse until they would kill each other. Eventually all the male Alicorns killed one another, leaving only females and thus, nopony to mate with."
Celestia added, "And when it comes to Alicorn sisters, once they come of age, they become very needy and lustful of one another."
Twilight asked, "So natural born Alicorn sisters...."
Luna replied, "Yes Twilight. The moment both sisters come of age, a special bond forms between them. They develop a bond of sexual desire for one another. With some it is only moderate."
Celestia interrupted, "But in the case of Lulu and I, it was extreme. There would be times when we would lock ourselves together in the same room........"
Luna interrupted, "Sister, there is no reason to go into specifics, I'm sure she knows enough already."
Twilight asked, "So incest is natural for natural born Alicorn sisters? You don't see anything wrong with it?"
Luna replied, "Twilight, unfortunately it is part of our biology. No pair of Alicorn sisters was ever known to not develop this bond. It is one of those things that once the bond develops, it is almost always eternal. The sisters will always desire one another, they will always be intimate."
Celestia added, "Even if they do find a mate at some point in their life, they will always desire their sister and find ways to have trysts with her."
When Twilight heard this, she was shocked. This was something that she simply could not wrap her mind around but then she realized one thing, that they weren't like that. She asked, "Celestia, Luna, if that is the case then why aren't you two like that anymore? Why don't you lust for one another anymore."
Luna replied, "We are no longer like that because when I became resentful of the ponies shunning my lovely night, I transformed into Nightmare Moon. In a sense, I became a different pony with a different mindset. Yes we were still biological sisters, but I became something else and that lustful bond was shattered."
Celestia added, "When she became Nightmare Moon, her desire went from being me to having full power over Equestria."
Twilight asked, "And I take it that when Luna returned from the moon, the bond did not return?"
Luna replied, "No, it didn't. While I did return to my former self, the bond never reformed and it will never return."
Twilight sighed in relief, "So I don't have to worry about you leaving me for Celestia or even if we were to become a committed couple, I wouldn't have to put up with you having to be intimate with Celestia?"
Luna replied, "No Twilight, you won't."
Celestia giggled, "Oh I wouldn't be so sure about that Lulu."
Twilight and Luna both sighed in frustration over comment. While Twilight knew that Celestia had not so pleasant plans for her marefriend, Luna was bound and determined to not only not allow that old bond to return but to get out of having to pay up in the way Celestia wanted her to entirely.