Q. Can a woman reside with her brother in law while her sister goes on a long distance trip?
Answer: Rasulullah (sallallaahu alayhi wa sallam) stated: “Brother in law is death.” Based on the correct meaning of this Hadith, it would be a sin to travel alone with him. Sayyidinah Uqba b. Amir reported Rasulullah (sallallaahu alayhi wa sallam) stated: Beware of getting into the houses and meeting women (in seclusion). A person from the Ansar inquired: (Oh) Rasulullah (sallallaahu alayhi wa sallam), what about husband’s brother, (what can you inform me regarding him, what should our relationship be like towards each other,) whereupon Rasulullah (sallallaahu alayhi wa sallam) said: The husband’s brother is like death. (Book 26, Number 5400, Imaam Muslim.) The word husband’s brother refers to all brother-in-law and sister-in-law situations alike.
Allah Certainly Knows Best.