Ruling on a wife uncovering her face before her mother’s husband

A man married a woman; is the woman permitted to uncover herself in front of her mother’s husband?
If a man marries a woman, all of his paternal ancestors become mahram (a relative one may not marry) to her. So, if her mother’s husband is her maternal grandfather, she may uncover her face before him. However, if her mother’s husband is the father of a daughter from a woman other than this woman, she may not uncover her face before him, because he is not a mahram to her.