How does he maintain balance when mother's weight doesn't lean/cooperate with him? 
Since he should be the one riding in front - and therefore managing the moveable handlebars, it shouldn't be that much of a deal (other than the risk that his brother falls off). The machine is so big and heavy that most turning is managed using the gyroscopic forces of the turning front wheel, and pushing the handlebars, rather than trying to force the bike into a turn by shifting body weight alone.
Racing bikes are a little different. Nonetheless, I almost learned this lesson in a very painful way when I first started riding a bike.
