All his brother has to do is sit upright, really. The wheels have a gyroscopic effect above a walking speed which helps stabilize the bike in motion. Getting the bike to turn is a matter of countersteering technique to get the bike to lean in the desired direction, and if his brother doesn't lean (either way) it makes the turn easier, same as with any passenger.
Exactly.
When I first started riding, I didn't "buy" the countersteering technique until one day when I was up at speed on a rural highway, and heading into a curve to the left. As I entered the curve, I started getting closer and closer to the right-hand shoulder, and try as I might, I couldn't "lean" the bike to the left to get it to turn. Finally, as the wheels hit the gravel shoulder, I let off on the throttle and out of desperation decided to try the countersteer technique. Instantly, the bike turned to the left and I was back on the pavement and out of immediate danger. All I had to do was to push the left handlebar ever so slightly forward, and the bike instantly responded.
