The sun lamp source will always cast a hard shadow because of what it's modeled after. The sun. The sun creates a hard shadow in the real world because if you look at it in the sky, it's quite small. It's more like a single light bulb. When creating a diffused light, avoid the sun lamp at all times. use environment textures instead, and use area lights on the walls to mimic/augment the reflections that are coming from the white walls. This way The whole scene will be lit instead of having all shadows be pitch black.
Interior scenes are difficult to properly light due to the amount of light bounces that Blender needs to calculate. Try to be creative using these 'artificial' lighting techniques until it looks right.
Also, the bright area in the back is very important. It looks like a wall that's getting a lot of sunlight through a window. This creates a lot of reflections that reach all the way into place where the camera is. You can use a sunlight to create that hotspot, but make sure the walls are blocking the light in other places.
Hope this helps!