Answered by Shaykh Rami Nsour
Question: One year ago I reverted to Islam and 3 months ago I got married to a Muslim woman. My parents are not Muslims. My mother’s husband is not my real father, but they have been married for 20 years. My question is if my wife is obligated to wear hijab in front of him, since he is not my real father?
Answer: Marriage only causes a non-marriageable bond (mahramiyya) between a woman and the real father of her husband. Thus, your wife would have to wear her hijab in front of your step-father. [Khalil ibn Ishaq, al-Mukhtasar]
source : http://seekersguidance.org/ans-blog/2013/06/30/does-my-wife-have-to-cover-in-front-of-my-stepfather/