Igros Moshe1(E.H. 4:65.11): Concerning the permissibility of a brother and sister being secluded together alone occasionally - whether this also applies in a situation where they are living in their parents home which therefore means it a fixed arrangement. When they are alone together in the house is this considered on occasion and therefore permitted or since he is in fact living with her in a fixed manner it is not permitted for them to be alone - even though the seclusion is only occasional. It seems that the practice of the world is to permit this. Even when the parents have only one son and one daughter who are old enough to be obligated in mitzvos, they leave them alone in the house for hours even at night. No one is concerned about the issue of yichud in these circumstances for brother and sister - even religious people. It would seem that this leniency is implied by Rashi who writes (Kiddushin 81b) that "brother and sister can be secluded together occasionally but he does not dwell with her constantly in the house".... this implies that the permissibility of seclusion with a sister is even when they dwell together for an extended time. However the prohibition of seclusion with his sister is only if they are alone constantly together but if it is only occasionally that they are alone then it is permitted even when they are living together for an extended time. That is the reason for the widespread leniency in this matter. However there is no question that they are not to be left alone in the house for an extended time at nights on frequent occasions until this is viewed as the normal situation - and not just an occasional occurrence. In this matter it is not possible to specify the exact number of hours or times that it is prohibited. Rather each one needs to recognize in his sons and daughters what is appropriate.