Other mistake: When young Vito Corleone shoots Fanucci in the chest at the doorway to his apartment, in a continuous shot, a wound/blood bursts from Fanucci's white vest. When Fanucci proceeds to tear his vest open seemingly in disbelief he has been shot, there is no entry-wound, blood or hole in his white shirt directly under where the wound appeared in his vest.
Other mistake: When Paolo is shot dead, there's blood dripping down the rocks he's lying on, but there shouldn't be. The rocks are under his shoulder, and he was shot in the back, and a little blood dripped out sideways. And there's no blood on his shirt where his body is touching the rocks.
Other mistake: When Michael visits Pentangeli's house, Pentangeli closes the double door to the study. There is a dead bolt handle on the right hand door. There is no deadbolt coming out of the door side, nor a striker plate on the opposite door.
Other mistake: When Hyman Roth's assassin runs at the airport, he is turning around with one (of two) bullet holes already in his jacket before the sound of any gunfire. (03:10:50)
Chosen answer: They helped him escape Don Ciccio's death warrant. He likely loves them like parents, and this would make them the childrens' grandparents.
Captain Defenestrator