Celestia had suspicions over her sister's strange behavior and after flying to Ponyville, she discovered what it was. She discovered that Luna's strange behavior and altered schedule was a result of her going to visit Twilight every night. She landed on their balcony to find them groping one another, kissing and moaning. She couldn't help but smirk watching them as a memory from long ago came to her mind.
She laughed, "So Luna, I thought you weren't seeing anypony."
This caused the two to pull apart and look up at Celestia in shock. Celestia asked, "So Luna, how long has this been going on?"
Luna replied, "It has been going on for two weeks."
Celestia said, "I see, no wonder you suddenly went from being depressed to being happy two weeks ago. No wonder we haven't had a good talk in two weeks."
Before Luna could reply, Twilight interrupted, "Celestia, please don't be mad at her. She was feeling depressed, she came to talk to me and while we were talking, we discovered that we have wanted to be together for a long time."
Celestia sighed, "Oh and how long have you two wanted to be together?"
Twilight replied, "Well I fell for her when she came to Ponyville on that first Nightmare Night after her return."
Luna added, "And I fell for her back then." She saw a look of disapproval forming on her sister's face and continued, "But we didn't pursue anything because we feared of what thy reaction would be."
Celestia replied, "Well that is good. Back then I would have been extremely angry over such a relationship, I would have likely banished you." She cast her gaze at Twilight, "And I likely would have made you move back to Canterlot."
Both mares asked "Seriously?"
Celestia replied, "Yes seriously but I've mellowed out a lot and besides, you're both grown mares and you're both princesses. I'm happy for you." She smiled at Luna and continued, "While I am happy for you, that doesn't mean you don't have to pay up."
Twilight asked, "What do you mean by 'pay up?'"
Celestia giggled, "Should I tell her Lulu or do you want to?"
Luna got a nervous look on her face as she knew exactly what she was talking about. She gasped, "Tia, that bet is over 1,500 yeas old! It certainly canst still be valid!"
Twilight asked, "Lulu? Tia? What bet?"
Celestia laughed, "Come on Lulu, you know there was no expiration date on that!" She calmed down and continued, "Now are you going to tell her or do I have to."
Luna protested, "Tia, thou canst tell her! It would be most embarrassing!"
Celestia laughed, "Sorry Lulu but your marefriend has to know."
Twilight asked, "I have to know what?"
Celestia giggled, "The bet! When my sister and I were younger, a friend of ours had her heart broken by another mare and this caused us to decide against ever pursuing romance of any sort. However, being the mischievous fillies we were, we decided to make a bet."
Luna added, "We made a bet that eventually one of us would cave in and pursue romance."
Celestia triumphantly added, "And now since Lulu lost, she has to make me happy in any way I want for a whole month!"
Twilight asked, "What is with this Tia and Lulu suddenly?"
Celestia replied, "When we were younger, we gave one another pet names."
Twilight asked, "Why would you give one another pet names?"
Celestia replied, "Because of the things we did together." She began to giggle, "And for the next month, it will be just like old times, won't it Lulu?"
Luna sarcastically replied, "For you!"
Celestia laughed, "Come on Lulu, you liked it then and I'm sure you still want it!"
Twilight interrupted, "Ok Celestia, what do you mean that Luna has to make you happy any way you want for the next month and what do you mean she liked it?"
Celestia giggled, "You know Twily, join Lulu and I and you can find out!" She leaned down to Twilight and whispered, "I hope you do because then I can show you all the ways to get Luna to submit to you entirely."
Twilight gasped, "You mean you two?"
Luna replied, "Yes Twilight, we do."
Celestia interrupted, "Enough of all this small talk. Come on you two, let's get back to Canterlot. I really need a release and I'm not about to let Lulu off the hook."
Twilight asked, "But what about Spike? I can't just leave him home alone for a month! I can't just ignore my royal duties for a month and I can't just abandon my friends for a month!"
Celestia replied, "Don't be such a worry wart Twily. Everything will be fine."
Luna replied, "Twilight, please come." She stuck her bottom lip out and continued, "Please Twilight, I love you and it would mean the world to me if you did this."
Twilight sighed, "Alright Luna, I'll do this for you."
As the three took off for Canterlot, they all were experiencing different emotions. Celestia was beyond thrilled because not only did she win the bet, but she would get to have the type of fun with her sister that she loved so much so long ago. Luna was nervous about what Twilight thought about this. She was worried that Twilight might find it so horrible that she would dump her. Twilight wasn't sure what to think. While she loved Luna dearly, she never imagined that she and her sister would engage in such activities. To her, incest was wrong but a nagging voice in the back of her head told her that perhaps there is a reason they do that and to keep an open mind.