Q) Can a man be secluded with his sister and another unrelated lady?
A) The Shulchan Aruch (Even Ho’ezer 22:5) rules that a man may be secluded with other ladies, as long as his wife is with him. The reasoning is that the presence of his wife will act as a deterrent from impropriety.
It is not clear from the Shulchan Aruch if this leniency – that one’s wife can act as a shomeres – is specifically applicable to one’s wife and not to other relatives with whom there is no issur yichud; or maybe we can extrapolate from the case of Shulchan Aruch and apply the leniency to any female relative with whom there is no issur of yichud.
Rav Moshe Feinstein in Igros Moshe (Even Ho’ezer vol. 4; response 65:5) discusses this point, and concludes that other female relatives with whom there is no issur yichud – e.g. one’s mother or daughter – can also act as shomros and permit a man to be secluded with his relative and other ladies who usually pose an issue of yichud.
Since according to many Poskim a man may be secluded with his sister – specifically on a short term basis, one’s sister can also act as a shomeres and permit a man to be alone with her and other women with whom there would usually be a prohibition of yichud.
The Tzitz Eliezer (volume 6, siman 40, perek 14) also discusses this situation at length, and quotes the views of various Poskim who rule in this situation. The Tzitz Eliezer concludes that since this situation of yichud is only ossur mid’rabonon and not min hatorah one can rely on the lenient opinions and rely on the presence of a female relative (such as a mother, daughter or sister) to permit yichud.