As DT said, Fredo, although he might have been left out of things by Sonny as well, wouldn't have been as resentful toward an older brother as he would by a younger one.

Also, no matter what Fredo did, Sonny never would have ordered a hit. He couldn't even bring himself to kill Carlo. Underneath all his bluster, Sonny was a bit of a marshmallow. He never could have ordered the death of his brother.