Yichud of brother and sister

Igros Moshe (E.H. 04:064.3): Yichud of brother and sister 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