Is my step-father Mahram for me?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The stepfather is Mahram of his stepdaughter.

So, there is no objection to a woman removing her Hijab in front of her stepfather.  Allah Says (interpretation of meaning) while enumerating the female Maharim (non-Marriageable): {….your stepdaughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters), - ….}[4:23].

Allah knows best.