Is it Yichud for a 14 year old girl to sleep over night at her uncle’s house, when the only ones home are her uncle, his 7 year old son and 11 year old son?
It is permitted. The reason being that during the day two men with one woman is permitted. At night though since people are sleeping, there is a need for one extra shomer, therefore three men and one woman is permitted.
Rema E:H 22 -5.