What is the permissible dress for a woman in front of her sister's husband, and is it permissible to wear wide pajamas in front of him, and what is the ruling on her staying overnight in her mother's house while the sister's husband is also staying there?

The sister's husband is like any other non-mahram (a man not related by blood or marriage in a way that would prohibit marriage). Therefore, it is forbidden for a woman to uncover herself in front of him, and it is not permissible for her to appear before him in what is known as "pajamas."

It is permissible for a woman to gather with her sister's husband in one house if privacy (khalwah) is absent, due to the presence of a mahram or a relative (dhū raḥim), along with adherence to complete lawful hijab, and avoiding excessive informality in interaction and conversation, unless out of necessity or need.