Other than Brand, and Dworkin in his latter years, I don't think any of them are insane. They are all a product of their upbringing and the ever present sibling rivalry over the throne. Their "couldn't care less" attitude in some respects is down to their perspective. If you're capable of living 10,000 years in the absence of assassination then what happens over a 60 year lifespan for an average human is not of great consequence to them.
Whilst I agree that the books are coloured from Corwin's perspective, his outlook was radically altered as a result of what happened to him, the attempted assassination, the black plague which caused him to completely lose his memory of who he was, and then the many lifetime he was forced to live there after with no explanation as to why. It was only Eric's decision to try and assassinate him again that caused him to remember his past when he awoke in the asylum. If Eric had just left him alone then the story would have been different.
This also shows another aspect of some of them, Eric was paranoid with respect to Corwin despite the fact that Corwin was potentially indifferent to him, but then Eric was a little paranoid any way. he viewed the throne as his by right and nobody else was going to have it.
Gerard is just a typical bully, and in many ways so are Julian, Caine , and Fiona.