The Girl Who Grew Wings by Anna Waterworth

In the citadel of Appollis, the Gods bestow single gifts on a chosen few. Icari has always known she's a Healer, while her twin sister, Sephie, is cast as an Embalmer, despite showing early promise as an Alchemist - a secret the sisters keep to themselves for possessing two skills is punishable by death.

While Sephie learns how to wrap the dead, Icari eases the suffering of others, including a charismatic enemy prisoner called Caszeil. And it's to Caszeil that Icari turns when demons rise up from the Underworld and kidnap her sister. With his help, perhaps she can rescue Sephie. Even if this means growing wings - and flying in the face of the devil himself . . .