Here's a breakdown:
Rodrick: Greg and Rodrick are constantly at odds. Rodrick is older, bigger, and often bullies Greg. Greg sees Rodrick as a nuisance and a source of embarrassment. He does sometimes look up to his brother, though, and they occasionally show flashes of camaraderie.
Manny: Manny is the youngest brother and Greg finds him annoying. He is often frustrated by Manny's immaturity and his tendency to get into trouble. Despite the frustration, Greg shows some protectiveness towards Manny, often trying to help him out of sticky situations.
Overall, Greg's relationship with his brothers is characterized by a mix of annoyance, rivalry, and occasional affection. They are, after all, brothers, and they do seem to care about each other, even if they don't always show it.