A husband’s stepfather is not a Mahram to his wife because there is no blood or permanent marriage ties. So it is impermissible for you to take off your Niqab in front of him.

If the wife’s father was breastfed by his sister and it was five full breastfeedings within the first two years, then he becomes a son to her through breastfeeding, and her children, both males and females, become siblings to him through breastfeeding; they become paternal uncles and aunts to his children through breastfeeding. Based on that, your wife may shake hands with the sons of this woman and uncover her hair in front of them, because they are her paternal uncles through breastfeeding.

The wife’s sister is not a Mahram to her sister’s husband, so it is not permissible for him to look at her, or be alone with her, or to shake hands with her.