What's the Name of That Book??? discussion

I read this book while on a book binge around 2013-2017. Google wasn't very helpful.

It starts with a girl following a lonely boy who she goes to school with. She follows him to the local elementary school, where he picks up his little sister, and eventually, they start talking. And the girl finds out that the boy's mother is missing.
At night the boy tells his sister a bedtime story about a young princess sailing in the ocean, looking for a new home because her island was drowned in the sea.
Along the way, they visit other islands, where they pick up different characters, like the answering man and the questioning man. One answers unasked questions and the other answers questions that are never asked.
As the book goes on, the girl realized that the answering man answered some of the questions asked by the questioning man, but not in order, and it is eventually discovered that the missing mother is actually dead and that the boy is the one who killed her, keeping the gun hidden above the bathroom mirror.

The Storyteller

I think that's it! Thank you so much!!!

Welcome, glad to help out!