"Kill the thief stealthily" is a mission objective in The Other Brother
Walkthrough[edit]
Thief 1
The thief will begin approaching from the North. Look for the marked target. Hit him with a Sleep Dart if you are on a rooftop then knock him out. Otherwise move in for a low profile assassination from behind(or just dart him). The more discretion shown, the easier it is to avoid the guards' notice.
Thief 2
As the brothers pass the gallows another thief will appear. He lurks in the Northwestern part of the gallows, looking for his target. Move to a clean line and fire and use the Dart Gun to incapacitate your target. it likely will help to wait for him to reach the edge of the gallows to hit him with the dart.
Thief 3
As the brothers continue past the gallows, take to the rooftops on the Western side. This will ensure you are in position for a quick aerial assassination when the next Thief appears