Answered by Shaykh Amjad Rasheed
I read in one of your previous replies that [h: a man’s] stepfather (lit. “mother’s husband”) is not considered an unmarriageable close relative (mahram) of his wife. Is a stepfather considered an unmarriageable close relative (mahram) of his [h: own] stepdaughter (i.e., his wife’s daughter [h: from a previous marriage])? I am a Muslim woman (alhamdulillah) and accepted Islam recently but my mother and her husband are still non-Muslims. Does the fact that my stepfather is non-Muslim affect whether or not it is obligatory for me to observe the rules of hijab in front of him?
After consummating his marriage with his wife, the stepfather is an unmarriageable close relative (mahram) of his stepdaughter irrespective of whether he is Muslim or non-Muslim. Allah Most High says, “[Forbidden to you are your mothers, your daughters, your ,] and your stepdaughters who are in your care and are born of your wives into whom you have gone in ” [h: (4:23)]. It is therefore permissible for a Muslim woman to uncover part of her body (such as the head, hands, and feet) in front of her non-Muslim stepfather unless she thereby fears suspicion, such as if it is well-known that the stepfather commits fornication (fahisha) , in which case she must conceal her body from him. Similarly, if the stepfather believes that it is permissible to marry unmarriageable close relatives (mahram), she must conceal her body from him.
(Translated by Hamza Karamali)
 Shaykh Amjad notes: “What is meant here [h: by fornication, Ar. fahisha] is fornication and its usual preliminaries, regardless of whether [h: the man does it] with unmarriageable close relatives or anyone else. What is meant, then, is that he is not upright of character so that she fears fitna.”