Is the hijab obligatory for your wife in front of her sister's son by rada'a (milk kinship), and for her sister's daughter in front of you, knowing that they were not nursed by your wife's sister?

The questioner's wife is a stranger to her sister's husband's children, so she must observe hijab (definition) in front of them, as they are not her mahram (definition). The questioner is a stranger to his sister's husband's daughter, so he is not permitted to be in khalwa (definition) (seclusion) with her, nor is she permitted to uncover her face in front of him. As for the children's father's wife, she is not required to observe hijab from them, because she is one of their mahram, based on the Almighty's saying: "And do not marry those [women] whom your fathers married, except what has already occurred."