DY, fair point
Some more information re R Moshe and siblings living together.
Slight intro, R Moshe differentiate incestuous relationships between father and daughter versus siblings. There is never an issue with yichud between father and daughter Whereas, siblings can have an issue.
(the difference is that father/daughter never WAS a Y”H. There used to be a Y”H between siblings and can become a problem if they love together on a permanent bases and become very comfortable with each other)
The SA states that it’s mutter for siblings to live together on a temporary basis and parents live with their children (dad and daughter) on a permanent basis. There is a dispute among the poskim what what constitutes temporary basis.
R Moshe holds that the temporary basis means more than 30 days. After 30 days it’s forbidden to live together. (taking a one day brake doesn’t help)
He goes on to say, that if people suspect that the siblings have a permanent residence together, then the 30 day limit doesn’t apply and they must separate immediately.
An example of siblings living together in a permanent basis is two siblings visiting a city for a period of time, may not rent an apartment even for less than 30 days since the time they do live together is on permanent fashion (they’re not “visiting” each other)
Siblings learning together in Israel (the boy in yeshiva, the girl in seminary) may never rent an apartment together
There’s absolutely no issue with siblings living together long term, permanently, if the parents are living in the home as well. There absolutely no problem with yichud.
Lastly, he states that if parents are taking a long trip, such as a trip to Israel, it’s better that they don’t leave a brother and a sister alone and RECOMMENDS leaving some kind of babysitter. He specifies this is only in a case where there are exactly two siblings living at home, a brother and a sister and no one else.
(my two sense: from R Moshe’s language it seems that the problem with siblings living together, is becoming too close and initiating a forbidden relationship, versus any kind of abuse.)