Note further that if the player's previous route went through the Trade Route Tunnel and disturbed the Zombies before Sidorovich would have contacted them; the side mission will not be given and it is thus rendered unobtainable. (Logic - the zombie population was disturbed)
There are important, non-trivial consequences that will determine how the Storyline unfolds depending upon the player's treatment of the occupants of the Sin House.
The Sin House where Fox is held prisoner, is essentially in the same location as in the Base Game, (but looks slightly different)
Inside, the player will find the Sin members holding him captive.
Two with Kalashnikovs are standing in the room with Fox kneeling in POW stance, while three others are sitting in an adjacent room speaking to each other in a strange language that is not Russian like all other stalkers' ambient talking is.
In order that the player completes mission peacefully, they simply must speak with the standing Sin leader on Fox's right, and follow instructions to get "that" from Sidorovich.
Alternatively, the player may opt for one or multiple blood-baths if they are feeling cranky, feisty, or bigoted towards the Sin faction members' awful appearance.
Firstly, if the player is feeling up for a killing spree, they may choose to kill soldiers under collapsed part of the bridge.
Whether or not the player decides to kill the Military under the Bridge Checkpoint, they also have the option to off the Sin squad holding Fox inside house.
Note, that although this obviously will lose points for the player with the Sin faction, killing them before even speaking with them is one of the conditions that may need to be met for the option to join Duty later on.