I don’t mean to give a final psak, however Yichud with a sister is permitted on a temporary basis, it is forbidden on a permanent basis.
What is the cutoff point is subject to dispute. There are those who hold it is thirty days, Reb Moshe zt’l disagrees and holds it is for an extended period of time, even if it is less then thirty days.
As for the fact they are in different rooms, the consesus of poskim seem to hold that as long as there are shared facilities, for example a bathroom it would be prohibited since they must leave their rooms.
However if each room had their own bathroom and the doors were locked then depending on other factors there would possibly be room to be leniant.
As was already noted the above is strictly for informative purposes and for an actual psak please consult a competent Rov who is knowledgeable in these Halachos.