In the first one, to have a brother, is a permanent state of existence. You either have a brother or you don't. In the second one have describes the action to eat dinner, to have dinner. This is an action so we can say "I'm having dinner.
In the first one, to have a brother, is a permanent state of existence. You either have a brotheror you don't. In the second one have describes the action to eat dinner, to have dinner. This isan action so we can say "I'm having dinner.