In most cases you need a letter.
Mexican law requires that if only one parent (or grandparent, or other guardian) is accompanying a minor under 18 into Mexico, they must bear a notarized letter from the child's other parent (or guardian) granting permission to enter Mexico with the child, including the dates of travel, the accompanying adult's name, contact information, and a notarized signature.
Your stepdaughter will likely need a letter that is authorization from both her birth mother and father.
We had a similar issue with a guest who is attending our wedding with her kids (their father is deceased). She was told to carry a copy of the death certificate...