What is the difference between patronizing your favourite store and patronizing your little brother?

Patronizing a store means to be a customer of, or to go there frequently. It's more like being a "patron of the arts." The idea is that you are supporting something.

Patronizing a person is to be condescending to... to pretend to be nice, but in a snobbish way, like someone is below you, or not as good as you are.