Illyria won the World Fantasy Award for best novella back in 2008, when it was published in the UK. Now, I have nothing to do with the World Fantasy Award (which is probably a good thing considering my general apathy for the genre), but looking at some of the other winners, this one doesn't quite seem to fit.
Madeline and Rogan think of themselves as two halves of the same soul. Children of identical twins who share a birthday themselves, they are inseparable as children - an intense friendship that grows into an intense love as they enter their teen years.
Multiple generations of their large family all live on the same block, all descended from a great stage actress, though Madeline and Rogan are the only ones who have retained a love of theatre and performing, encouraged by their great aunt who appreciates the art of theatre, though doesn't seem to perform herself.
When cast as the stars of Twelfth Night in a school production (Madeline as Viola, Rogan as Feste), Madeline and Rogan must begin to confront their separate talents and where they might lead them, even as their relationship grows in intensity in the face of adversity.
Notice how in that description there isn't much that would really fall into a "fantasy" story? It's more like what one of my college professors called magical realism. This is a rather straightforward story about two extraordinarily close cousins who face some adversity because of their peculiar talents and relationship. The fantasy/magical element comes from an apparently enchanted miniature theatre they find embedded in the walls of the attic, where the lovers escape for their illicit trysts.
While published as a YA novel, the only resemblance this has to most YA is the ages of the protagonists - but in some ways I never felt like I was reading about teenagers because Madeline is telling this story as a middle aged adult. I had only a tenuous connection with her during the high school phase, but I totally lost it once she skipped ahead to briefly show her life as an adult, and how her and Rogan's decisions from back in their high school days had panned out as adults.
The romance between Madeline and Rogan is heady and impulsive - and yes, they are cousins, but really, we're still grossed out by that these days? Their familial relationship isn't fetishized - rather Madeline sees them as two halves of one whole, and it's the most natural thing in the world for them to come together. Maybe if someone in the extended family modeled a healthy relationship, or tried to speak to the kids frankly and without judgment, things would have turned out differently, but as-is, the romance was the least-problematic aspect for me.