The Iron Traitor by Julie Kagawa

In the real world, when you vanish into thin air for a week, people tend to notice.After his unexpected journey into the lands of the fey, Ethan Chase just wants to get back to normal. Well, as ‘normal’ as you can be when you see faeries every day of your life. Suddenly the former loner with the bad reputation has someone to try for – his girlfriend, Kenzie. Never mind that he’s forbidden to see her again.But when your name is Ethan Chase and your sister is one of the most powerful faeries in the Nevernever, ‘normal’ simply isn’t to be. For Ethan’s nephew, Keirran, is missing, and may be on the verge of doing something unthinkable in the name of saving his own love. Something that will fracture the human and faery worlds forever and give rise to the dangerous fey known as the Forgotten. As Ethan’s and Keirran’s fates entwine and Keirran slips further away, Ethan’s next choice may decide the fate of them all.