If a husband will be out of town for over a week is it permissible for the wife’s unmarried brother to come for shabbos or would this pose a problem of yichud?
Answer:
It is fine to come for Shabbos.
Sources:
There is no prohibition of yichud for brother and sister (Beis Shmuel, Even Ha-Ezer 22:1).
Nonetheless, it is not permitted for them to live together on a permanent basis.
There are differences of opinion as to what constitutes a “permanent basis,” but even the most stringent opinions give a time of three days, so that coming for Shabbos will not be a problem.
See Otzar HaPoskim 22:2 in the name of Tzuf Dvash (the most stringent opinion). For the more lenient opinion of thirty days, see Imrei Yosher Vol. 2:43, Shraga Hameir Vol. 4:99, and Toras HaYichud 2:19.
Some define the situation based on more flexible categories of “visiting” or “permanent resident,” without giving fixed times. Certainly, coming for Shabbos is only “visiting.”
