The Smoking Mirror by David Bowles

Carol and Johnny Garza are 12-year-old twins whose lives in a small Texas town are forever changed by their mother’s unexplained disappearance. Shipped off to relatives in Mexico by their grieving father, the twins soon learn that their mother is a “nagual,” a shapeshifter, and that they have inherited her powers. In order to rescue her, they will have to descend into the Aztec underworld and face the dangers that await them.