What is a woman allowed to wear before her brother-in-law? Can she wear pajamas before such a person, given that it is loose-fitting? What is the sharia ruling if she spends the night at her mother's house while her brother-in-law (her sister's husband) is spending the night at the same house?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Your sister's husband is like other non-mahram (permanently unmarriageable) men, whom a woman is forbidden to display herself before. The only difference between him and others is that he is forbidden to marry his wife's sister as long as he keeps his wife in wedlock or while she is observing her ‘iddah (post-marriage waiting period) after divorcing her. This, however, does not affect the ruling regarding the way that you have to dress before him, not being in seclusion with him, and so on. He is like other non-mahram men, and a woman has to appear before him in the same way that she should before other non-mahram men.

This indicates that a woman is forbidden to appear before non-mahram men, even if they are her sisters' husbands, while she is wearing the so called pajamas.

Being in the same house with her sister's husband, whether at night or daytime, is permissible as long as one of her mahrams and kin is there with them. it is also required that she observes other Sharee'ah regulations like avoiding being in seclusion with him; wearing the Sharee’ah-approved hijab; and avoiding informal treatment with him; rather, it is better not to talk to him at all, unless there is a necessity or need. We have already mentioned the scholars' statements regarding the prohibition of a man talking to a young non-mahram woman.

Allah knows best.