I think the possibility of a brother being disloyal was a concept foreign to Michael (until it was almost too late). That whole scenario was written especially for Part II (which also included Michael's dark side slide) and Vito wasn't around for any of it so he couldn't have foreseen any of this happening.

Even though Puzo co-wrote Part II, there was so much added especially for the sequel that it's really hard to to compare the characters' differences (between Part I and II).

Welcome to the boards, Primo.