Ughhh... I was sketching, see...and then I was sketching too long. Yes, yes, this isn't what a palace would look like. It's not what any place would have looked 1,000+ years ago, either.
Anyway, the idea is that they only see each other awake at dusk and dawn, since Luna and Celestia were born to reign night and day respectively, and the twilight hours don't really belong to either exclusively. So Tia nodded off shortly after relaxing in the company of her newly-awoken sister. I thought they might enjoy being alone in the house as well, so Luna here welcomes the night while her sister sleeps.