Hex You (Sisters of Salem #3) by P.C. Cast
Twin sisters, Mercy and Hunter are witches, direct descendants of the Goode family, the founders of their town. After the murder of their mother at the hands of a foul demon, they have become the protectors of the Gates to different underworlds–ancient portals between their world and realms where mythology rules and the darkest of creatures exist.
Mercy and Khenti are trapped in the Ancient Egyptian Underworld and need Hunter’s help to escape. But while Hunter searches for a way to save them, other evil threatens Goodeville. Amphitrite is still looming–and she wants vengeance against Hunter. With the gates rapidly weakening, Amphitrite lures out a deadly creature and sets it free on the residents of Goodeville. It will take everything in Mercy and Hunter’s power to stop the goddess and seal the gates once and for all.