Thirteen by C.M.Rosens

Katy Porter is the thirteenth child of a thirteenth child in an inbred family of eldritch horrors, and her own eventual metamorphosis will change her into a creature that hungers for her family's flesh.

All Katy wants to do is finish college and go backpacking with her mates, but as her changes creep ever closer and her protector is horribly murdered, that's not looking too likely.

Katy needs allies, but she's stuck with her oldest brother, a drug-addled playboy, and her eyeball-eating cousin, whose idea of protecting her involves abduction, dark rituals, and encouraging her homicidal side.

If anyone is going to survive Katy's transformation, scores need to be settled and fears need to be faced - and Katy is not the only one who needs to face them.