Great question, Olivant!

I've never given it any thought. Do we just assume that they accepted their deferments as a "gift" from their father, and did not feel the same need to rebel as their younger brother did?

Now, Sonny was already married, with children, at that point, so it's understandable that he wouldn't want to go, but Fredo would have only been about 26, and single, after the Pearl Harbor attacks. I guess he didn't want to miss out on the cocktail waitresses.