The Darkest Fall by Ripley Proserpina

Can a demon be redeemed?

Rational Lucia believes in what she can see and touch, preferring to leave the curses and fortune telling to her old-world relatives. Meeting Delia, a desperate half-human, half-demon child, who needs someone to keep her safe challenges Lucia’s logic.
Armaros spent his eternal life mired in guilt. Choosing to live according to his desires and whims, he forced his sister to fall from grace, damning them both. When she vanished, abandoning him and all they’d built together, he pursues. The last thing the Fallen expected to discover was the impossible existence of her child and the fragile human intent on her protection.
Thrust together, Armaros and Lucia must flee the forces of Heaven and Hell. Despite the insurmountable odds stacking against them, can they accept that fate put their paths on a collision course? Or are they doomed to fail, losing not only Delia, but also the only other person in the universe who understands them?

*The Darkest Fall was previously published as Matched with a Demon*