Thicker than Water by Ellie Roth

Julian Jackson, demon hunter, doesn’t expect his latest mission to end with him accidentally adopting the demon he was supposed to capture, but tiny Desi immediately wraps JJ around her little finger—and wakes up all his brotherly protective instincts. Hiding her in the midst of the hunters’ headquarters is a challenge, but he’s determined to make it work… until another demon breaks in and abducts her.
Cassius Chin, demon, is dumbfounded to learn that the demon toddler he just saved from the evil hunter didn’t actually want to be rescued—and that Desi wants her JJ back. Meeting with the hunter to discuss joint custody over their jointly kidnapped daughter is a new low in Cass’s immortal life, and co-parenting her with JJ is an exercise in patience… and several other emotions he doesn’t want to examine too closely.