An Explanation of the Castle

In my first chapter, I described a bit of the mystery of the Castle that all the alicorns lived in. I wasn't able to fit a full explanation of how its magic works, and so to clear up possible confusion I'll write a blog about it here.

If anything comes up that I feel I need to explain outside the format of the story, I'll probably make a blog. Look for those if you're especially confused about something. If there isn't a blog written, the explanation is likely in the book, but probably hasn't been published yet. All will be revealed in its own time

Explanation

So, as we read in chapter 1, the castle is split into two halves. One half belongs to the day, and one half belongs to the night. While every alicorn lives inside this castle, the two main rulers (because despite it being a race of royalty there still needs to be a leader to keep the peace and order) have a magic affect on the castle.
Celestia's mother controls the sun and day, and her father maintains the night and stars. These two are obviously wed. Two rulers must be married to rule the castle, and it is a hereditary reception. That means that at least one alicorn born of the previous ruler must always take part as a main royal.
This doesn't mean that the siblings must marry one another to continue the line. This means that each can marry an alicorn of different line, and depending on whoever receives the throne, the castle will actually change based upon their talent.
Let's say for the sake of it that Starswirl's mother, who controls the weather, actually married Celestia's father. Ignoring the incredible weirdness of the situation, this would mean that half of the castle would belong to the night, and half would belong to the weather.
So despite the incredible power of the controllers of the sun and moon, they wouldn't always be the ones to be the main rulers of the castle. No incest here.
Now, Celestia's parents have ruled for a very very long time mind you, and honestly scarcely any of the alicorns recall the castle ever being anything but Night and Day.

Hope this explains some things a little better about the Castle.
Happy reading!