Well, sure, they were all a pretty nasty bunch, even Corwin himself. The question is one of perspective... they all viewed each other as possible resources. And when you have a resource (one of maybe 15 known) who can create worlds inhabited by millions of troops willing to fight for your cause just by walking to them, well... you don't just whack him over a little thing like some plan he might have to stab you in the back and take over once you've won.
Plus, there was the familial love thing going... they were all brothers and sisters, after all.