Before leaving to sell the pearl, Juan Tomas warns Kino and Juana to get the best price for the pearl, and tells him how their ancestors got an agent to sell their pearls, but this agent ran off with the pearls. Kino had heard the story told as a warning of punishment against those who try to leave their station. From this, we can infer that Juan Tomas wishes the best for his brother, but also understands that the buyers will atempt to cheat him. Some might believe that Juan Tomas' warning allude to his own greed and the hopes his brother's fortune will be shared. Others ight see it as the older brother assuming the younger won't handle things properly. I prefer to believe their conversation means that Juam Tomas is simply looking out for Kino.... family loyalty,