@Cornonthekopp At the end, the panel of the feet - those are Nox's feet, it's also why the Aunt and Uncle are so shocked. The Day Goddess is hiding her there.
I think she wants to kill her sister to become the only worshiped deity in their country. Especially since recently people have decided to start worshiping them more equally (since the Night Goddess Temple is going up soon.) She wants to get rid of her sister before she gets too much power.
The reason I think that their mother was probably a worshiper of the Night Goddess is because everyone called their mother "witch" and that's also how they call the Night Goddess. Also explains the protection the Uncle receives from the Night Goddess.
But that's all conjecture, lol.