Assalam wa alykum is the grand father's step brother counted as mahram? should a women cover herself completely in front of him? jazakallahukhair

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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

If you mean by the "grandfather's step brother" that he is the son of his father’s wife form another husband, then he is not a brother to the grandfather, and he will not be considered a Mahram to the children of the grandfather, except if there is a breastfeeding relationship between them.

However, if you mean that the person in question is a maternal half brother to your grandfather, not a full brother, he will be a Mahram to his brother's daughters and their female descendants. That is because they are forbidden to marry him by blood relation according to the consensus of scholars. In Al-Mughni, Ibn Qudaamah said: "The brother's daughters refer to every woman attributed to one's brother by birth; she is a Mahram to him whatever the kind of brotherhood may be, because Allaah The Almighty Says (what means): {Prohibited to you [for marriage] are…your brother's daughters.}[Quran 4:23]”

In this case, it is not obligatory for the woman to wear her Hijab in front of the maternal half brother of her grandfather; rather, he is allowed to see from her what a man is allowed to see from his Mahram women, i.e. parts which usually appear from them like, for instance, the face, the head, the feet and the arms.

Allaah Knows best.