Is it permissible for a 20 year old boy to sleep in the same room as his 18 year old sister when they go to eretz yisroel on vacation.. they will be staying in a room in somebodys house not in a hotel
It is permitted for a brother and sister to be b’yichud in a temporary setting. Some poskim say that if it is for less than 30 days. R’ M. Feinstein, and R’ Y. Kaminetsky zt”l held that it depends on the amount of time that would be considered a visit. Therefore it would depend on the amount of time that it is normal for people to visit from America to Eretz Yisroel.
There is another potential issue, that since they are in a foreign place people are not going to know that they are brother and sister and it looks like yichud, however if they are staying with a private family, and the people of the house know that they are siblings then that would not be an issue.
Aside from that they should be careful to remain tzniusdik even while they are sleeping.
Kiddushin 81b, Rashi Rosh and Ran ibid, Chochmas Adom 126-12, Aruch Hashulchan E:H 22-1, Dvar Halacha 2-4, Igros Moshe E:H 4- 64 (3), Emes L’Yackov E:H 22- ftnt. 7, Tzitz Eliezer 6-40 perek 20-7, Toras Hayichud 2-19.