A Book of Spirits and Thieves by Morgan Rhodes

This is a spin off of a series by Morgan Rhodes.  One sister goes to Mythica, leaving her modern world behind.  The other sister works to bring her back, learning all kinds of family secrets.  Solid characters.
The bad teen is solid.  I keep hoping he'll be a good guy at the end.  Who knows????  IF you like the series, enjoy this spin off.