What is the ruling on a woman staying in her sister's house to help her while she is pregnant, given the presence of the sister's righteous husband, visits from her brothers, and her sitting with her sister's children?

The sister's husband is a stranger to her sister, and it is not permissible for her to be alone with him or to show him her adornments, because her prohibition to him is not permanent, and what is forbidden is combining between two sisters [in marriage]. The Prophet, peace and blessings be upon him, forbade privacy with women and warned against it, as in his saying: "Beware of entering upon women," and his saying: "No man should be alone with a woman except with a mahram." Therefore, it is not permissible for the wife's sister to be alone with her sister's husband, or to uncover in front of him, or to show him her adornments. However, her presence to help them in the house without privacy or showing adornment is permissible.