I wondered why Vito didn't involve himself with Connie and Carlo, even though he was a traditional man.It is suggested that he doesn't really trust or approve of Carlo as he tells Hagen to give him a living but nothing more (This being one of the things that sparks Carlo's resentment).
Therefore, knowing his beloved, pregnant daughter was being beaten by a man he didn't really like all that much must have got to him. Sonny's reaction was more 'normal' in that he disregarded tradition and tried to help his little sister. True, she could be a pain in the rear at times, but come on!
Also, if he had got involved it could have avoided the death of Sonny. No need to go steaming along the causeway 'coz there's no more Carlo and Connie's safely in the compound?