The world of Red Dead Redemption 2 is filled with some colorful individuals that will ask Arthur for help in special, optional missions known as Stranger missions. The Oh Brother stranger mission is perfect if you're looking for a short activity.
The Oh Brother stranger mission becomes available in Chapter 4 after completing The Gilded Cage mission, which is the sixth story mission of the chapter. To begin the mission, you need to head over to Valentine, approach the General Store and interact with Proetus and Acrisius, a pair of twins who are arguing about who among them is the one most deserving to be with a woman named Helen.
As words cannot settle things, they decide to prove their love to her by showing who is the bravest of the two, and they will end up involving Arthur in their dispute. Follow them to the back of the store and shoot the bottles off their heads, starting with large bottles and ending with smaller bottles and the twins standing on a single leg. Shooting the bottles is very easy, but if you're not feeling too confident, just activate Dead Eye and let loose.
Once the third round of shooting has been completed, the twins will go away, dragging the poor Helen with them.
The second part of the Oh Brother mission can be started in Valentine once again, but only after a few days have passed since completing the first part. You can skip days by setting up camp right outside the town and sleep for the required amount of time.
Even though some time has passed since the shooting contest, the twins haven't cooled off at all, and they still want to prove how much they love Helen by doing something stupid. This time, they will prove their love by getting beaten by Arthur. When prompted, hit each of the twins in the face, in the stomach and then between their legs.
Like for the shooting contest, there are no winners. So the two drag Helen off once again, unable to show who is the one most deserving of her.
The third and final part of the Oh Brother stranger mission can be started by heading to Cumberland Falls, which is located to the west of Valentine. Like for the second part, you need to wait a few days before being able to begin the third part of the mission: just sleep for a while at camp and you will be fine.
As the twins feel that they haven't really shown how brave they are, this time they have decided to travel down the waterfall in a barrel. Arthur has to follow them on horseback until reaching the area marked on the map. After the twins' fall, however, things don't look particularly well, as they are not coming back.
Helen asks Arthur to go check what it happening: ride downstream until you reach the marked area on the map to see how the twins have finally realized just how foolish they have been. They also seem to have lost interest in Helen, something that leaves both the woman and Arthur puzzled.
After the final cutscene has ended, the Oh Brother mission has been completed for good.