Am I considered a Mahram to the wife of my foster son?
Yes, you are considered as a Mahram to her. Allah (Exalted be He) says:
...and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers
Also, it is authentically reported that the Prophet (peace be upon him) said:
Wet-nursing makes unlawful what consanguinity makes unlawful.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.