With regard to Yichud: All Halachic authorities agree that a brother may not "live" alone in a home with his sister, but there are different opinions as to what "live" means. Some (Imrei Yosher 2:43) hold that less than thirty days is permitted, while others (Shevet Halevi 5:201-2) hold no more than three days is permitted. According to Rabbi Feinstein's ruling [Igros Moshe Even Haezer 4:64-3], it all depends on the length of stay of a typical house guest. Thus, a sister who is visiting from a distant city may stay longer than a sister visiting from a nearby area, just as a guest from afar stays longer than a guest from nearby.

The above refers to simply being under the same roof together, not to sleeping in the same room. This is where the issue of Tzniut comes up. It is definitely not modest for a grown woman and man to sleep in the same room and should surely be avoided.