Right, all true, but nobody knew when Carlo would "touch" his sister again. Carlo never alerted anybody - now unless the Barzini's had Carlo's wire tapped, then they knew because Connie called the Corleone compound.

But in neither the book nor the movie, guilt towards Carlo is never implied - not until the final scene.

The first time you saw The Godfather, you knew that was going to happen? I didn't. I thought it was just coincidence. Carlo was gonna beat her anyway - it was in his nature - and eventually Sonny would leave the compound; guess Barzini figured it would only be a matter of time?