Is it permissible for a 14-year-old girl to stay with her uncle and his 3 daughters (ages 12, 9, and 5) for Shabbos?
Thank you for your question.
It is not yichud, since your uncle is together with three of his daughters. Even though it will be at night, and there is a need to have one extra person there to break the yichud, it is permitted.
Rema E:H 22-5.