"it would be good for the family / there was something in it for me" Fredo took in some Roth/Ola men who were then able to ease the entrance of the assassins onto the compound, and who also would be there to tie up loose ends if the hit failed.
"...killed by someone close to us" could easily have meant any of Fredo's men (even recent recruits) who were at the compound. It didn't necessarily refer to someone more intimate than that, even though it gave that impression.
Fredo realized all of this after the attempted assassination, hence his "you got me in deep enough already".
