I wonder if the Emperor actually had sex with Estrella's mom. Maybe she has some sort of magic that allows her to conceive a kid with a man without sex.
I hope the Empress doesn't come back and throw a wrench in the happy family. My guess is that with how delicate the Emperor is being with Estrella, he wouldn't have informed the Empress, or he would be more frantic about her return if he thought she would be mean to Estrella.