Fantasy book with an ice castle (or crystal). Two main characters (siblings), one is named Robin, does spells in sleep. Cover: blue. Read 2016.

[Apollo]

uhh sorry if thats like too vague but i know i read it in 2016 nd the cover was like #5FBCD3 colour wise.. (i forgot the drawing for the cover)

[Kris]

Apollo, is this book for kid, teens or adults?

Can you describe the fantasy world (e.g., medieval, modern day, etc.)? Are there fantastical creatures or only humans?