Why does Shichika's sister sit off to the side behind a column?
This is just my speculation from seeing your screenshots, as I don't remember the scene. But it feels to me that it is meant to symbolize that Nanami is not part of the discussion. Sitting far and facing away means she is not supposed to talk and anything she says can be ignored by those actually engaged in the dicussion.

This is understandable because it is Shichika who is head of the family. And moreso that Nanami is a woman. Because this is a culture where women were often not meant to have power and make decissions.