IDIOS is not the most normal way of indicating possession in the
second person but examples do exist (e.g., 1Thes 2:14, 4:11). The
fact that it is not normal would make it useful for drawing
attention to the idea of possession. It would be a less bland way of
stating it than OUK ESTIN SOU ANHR.
What I am drawing attention to here is the notion of semantic
marking. I am suggesting that the form of the statement: OUK ESTIN
SOU ANHR is the semantically unmarked way of indicating
possession. It does not draw particular attention to possession.
For this reason it probably does not provide enough information
to determine the nature of ownership which was the force of the
original question.
Clay Bartholomew
Three Tree Point