I don't know if ya'll know this or not, but there are two My Little Pony books out there. One being The Journal of the Two Sisters and the other being The Elements of Harmony. If you haven't gotten these books, you need to RIGHT NOW!
I not only wanted to plug that for my fellow bronies, but also bring something up. First a small synopsis. The Elements of Harmony, details the show, the creators, has lyrics and episode descriptions for each episode from Seasons 1-4. It is a companion book and a delightful read.
The Journal of Two Sisters however is what I want to talk about. It is the chronicles of both Princess Celestia and Princess Luna, before they were princesses and afterwards. Now in the Hearth Warming Eve episode of the show you learned that these two did not always control the day and night. Unicorns did this job. In the book it tells you that Star Swirl the Bearded always did this with the help of other unicorns.
This is where things get a bit interesting. Star Swirl chose Celestia and Luna to become Princesses. They were also already alicorns when he chose them. When they received their cutie marks, their marks matched the same as the Tree of Harmony, which also bears the same mark as Twilight's cutie mark.
Star Swirl was also using time travel, and thus he started to change in age appearance as a side-effect. This makes me wonder if the pony a lot of people call Doctor Hooves is actually Star Swirl, and is merely there to make sure Twilight is alright and to keep an eye on the Tree of Harmony.