What might lead kino and his brothe to believethis?What might lead them to be suspicious of the priest's sermon?

In your first question I am not sure what "this" means. As for the sermon, the priest warned that man should know his station in life and not try to better himself. This viewpoint allowed corruption to run rampant amongst the buyers and served to keep their prices at a minimum. Kino and his brother knew the sermon was intended for Kino to share his wealth because he is a common peasant.