I always thaought that if someone would enter the family business it would be Frank. Perhaps Frank would remember his grandfather coming home from the hospital in part 1 and the letter he wrote as well as the time he spent possibly around Clemenza and his father, this would draw him into the business. Sandra may have taken the kids and fled but one of the deleted scenes from GF2 was her bringing on the daughters back to get permission from Micheal to get married, or maybe I'm mistaken. At any rate with the potential temper of there father the boys would more than likely get into some trouble that would require Micheal Corleone to pull some strings.