Children’s fantasy novel where the people have gems inlaid in their teeth and must smile

The book was about a girl trying to protect her sister. I remember that it was a part of their culture that they had to smile all of the time and so people would get gems and stones set into their teeth. There are also people that could astral project and the whole society pretended that the sister being protected was just astral projecting. The sisters are forced to leave their home and the protective sister eventually teams up with a guy. I don’t remember much else, but I think the society the girls lived in was called the Lace or the Lost and there was a school for the kids who astral projected called the Beacon I think? The cover was mostly blue and showed a forest with the main sister peeking through the leaves. Thank you for helping!

The Lost Conspiracy by Frances Hardinge. Aka Gullstruck Island

Thank you so much! That’s it exactly!