Why are they almost all so unhealthy?

Hashirama? The guy let his little brother become a racist and there's no real indication that he died before Tobirama became Hokage, so maybe he could have stopped his actions. Being a good big brother also means educating and righting the wrongs of the younger brother. Tobirama abolished his brother's democratic system, so it seems he didn't have much respect for his opinions.

Hiashi? Honestly, if it wasn't for Itachi, he'd be the worst big brother ever. The guy literally let his twin and nephew be slaves. Hizashi literally had to die for his brother.

Sand's trio? How they managed not to kill each other off before Gaara's character was turned into a Naruto fanboy remains a mystery.

Urushi? Unfortunately, from what little we've seen in the manga, Kabuto acts more like an elder than he does. Kabuto is definitely one of the best brothers in the series. He may be a bad guy in other ways, but as a brother, he's the best. He only joined Danzo because he wanted to protect the orphanage.

Itachi and Sasuke? Do I really need to explain this one?

Madara and Izuna are the only ones with a healthy relationship. Madara did his best to protect Izuna and never mistreated him in canon ( unlike another Uchiha). Izuna and Madara are the only brothers to act normally (if we exclude the whole eye swapping thing, but even so, Izuna consented).