Your mother’s husband is a mahram because of marriage. There are four persons considered as Maharim (close relatives you cannot have marriage relations) for a woman via marriage:  Husband’s father, husband’s son, husband of wife’s mother, husband of wife’s daughter. However, the bar to marriage with the mother’s husband comes into effect only after the consummation (of marriage). In other words, no bar exists if the marriage is dissolved before consummation.
In conclusion, your mother’s husband is considered your step-father even though their marriage is no solemnized according to the rules of the Shari`ah. Since they are not Muslims, their marriage is deemed as long as it fulfills the requirements of the particular religious denomination to which they belong.  Islam recognizes the validity of marriages of the People of the Book as long as they conform to their acceptable norms and customs.