I am married, and i want to know if my stepfather is mahram to my wife.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The fact that this man is your step-father does not make him a Mahram to your wife; rather, he is a non-Mahram to her, and she is obliged to wear Hijab in front of him and not allow him to be in seclusion with her and the like. However, if there is anything which makes him unmarriageable to her, like being her (paternal or maternal) uncle for example, then, in this case, he is a Mahram to her.

For more benefit, please refer to Fatwa 85868.

Allaah Knows best.