Oh, Brother is one of the side quests available in Red Dead Redemption 2. This is a rather humoristic mission. Arthur can help Acrisius and Proteus in ending their fight over a certain woman.
The first meeting
This quest starts in Valentine. Wait for the marker to appear on the map. The quest givers are near one of the buildings.
Your task is to shoot objects placed on the quest givers' heads. Use the Dead Eye to be sure that your shots are accurate.
The second meeting
Wait a bit for the next quest marker to appear - this one will also be in Valentine. Here, Arthur has to hit both men a few times. Do that by pressing the melee button. Keep doing that until they have enough.
The third encounter
The third and the last meeting with Acrius and Proteus happens in the location of Cumberland Falls from the above picture. It is a large waterfall located west of Valentine. The mission marker will appear only after you approach Cumberland Falls and only if at least one day has passed since your last meeting with the brothers.
After the cut-scene, push one of the barrels. Then, go with Helen down the river and reach the place, where the barrels stop. Approach the men lying on the ground (screen above). The last cut-scene of this mission will commence.