Yichud of brother and sister

Igros Moshe (IV #64.1) Question Is there a problem of yichud of brothers and sisters?Answer While there is no natural lust between brothers and sisters that is not because of the physical reality but is the result of the action of the Anshei Knesses Hagedola to remove the lust between relatives so in reality there is in fact some lust and pleasure even though it is not as powerful as between a normal man and woman, they only weakened it. Therefore they are allowed to be alone together only for a short and irregular basis and even to sleep overnight in the same house. Is permitted but not on a fixed basis.  The same is true for his father’s sisters or mother’s brothers . This is true for Jews and non Jews.

Igros Moshe (E.H. IV #64.3): Question Yichud of brother and sister Answer The gemora (Kiddushin 81b) says that a person can have yichud with his sister and live with his mother and daughter. Rashi explains he is allowed to have yichud  with his sister occasionally but not live together with her constantly in a house. So what does occasionally mean? It is makes no sense to say to say it is once in 30 days we say for the beracha on seeing the ocean because the gemora only describes the time for berachos. Look at the brief rulings of the Rosh who says a person can live temporarily with his sister and permanently with his mother. In other words it needs to be clear that he comes to her house only occasionally as a guest and is ready to leave every day and not as one who comes to stay a fixed short time. Thus it is not the actual amount of time but rather that it transient and temporary. So even if he ends up staying for a year or two  or more and then leaves it is not consider permanent. Consequently  it needs to be clearly seen that he came to her house as a guest and not to live permanently which is prohibited even for a single night. As a guest he can sleep overnight even several nights. The same applies for his sister visiting him. Thus it is not a fixed time but the normal behavior of guests in that community. If there is any doubt it is forbidden and not like rabbinic doubts for which we are lenient. Regarding a brother and sister who are still living with their father and likely are very habituated and live closely together which was true even in Talmudic times and yet there were no concerns for yichud

Igros Moshe (E.H.IV #65.11): Question: Permissibility of a brother and sister being secluded together alone occasionally - whether this also applies in a situation where they are living in their parent’s 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. Answer: 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.