Welcome, Hank! Your English is just fine! And you join us with some distinguished company from the Netherlands.
In the novel, Puzo makes a subtle point about Luca Brasi: that he chose to fear Vito Corleone. Evidently, Vito interceded on behalf of Filomena, the midwife who delivered his illegitimate son, who was afraid of Luca for the reasons stated in the novel. Vito approached Luca, and Luca evidently was sufficiently impressed with Vito to pledge his loyalty. But I don't think their relationship was based totally on fear. Vito needed Luca's forcefulness, and Luca enjoyed Vito's patronage.
As for Michael's children: your example is one of many differences between the novel and the films. If you stay with these boards (and I hope you do), you'll find many more.