When Sonny is done 'visiting' with Lucy Mancini, he walks out of her apartment and tells his bodyguards "Save it for the library -- Come on, we got to go pick up my sister -- let's go."
The Corloene's were at war. Sonny was a hunted man who shouldn't have even been outside the Corleone compound.
Now why in the world would he ever jeapordize his sister's life by personaly going to pick up his sister and put her in his presence, out in public, during a mob war when his enemies were looking to kill him?
