Here is my 100% Achievement Guide for The Sinking City.
This is at the end of the 4th Main Mission "Father & Sons".
You have two choices here, go to Brutus and tell him that it was
Or go to Graham and tell him what needs to be done.
VERY IMPORTANT, when you get outside the room where Brutus is.. MAKE A MANUAL SAVE as this correlates with another achievement we need to do.
For this particular achievement, we need to go back to Brutus in the smugglers hideout and let him know that it was indeed
Once that achievement unlocks, reload your save but this time head to Graham and do what needs to be done!
Go to 3:41:50 in my part 1 video to see where and how this unlocks.
Hope This Helps!