Is my stepmother's father regarded as a Mahram (unmarriageable relative) to us and we and my mother can take off the Hijab (veil) in his presence?
The stepmother's father is not regarded as a Mahram to the daughters of his son-in-law from another wife. Accordingly, they are obliged to wear Hijab in his presence. (Part No. 17; Page No. 441) May Allah grant us success. May peace and blessings of Allah be upon our Prophet Muhammad, his family and Companions!