The sequence in the novel that you refer to builds up Michael's education into the ways of the Mafia during his Sicilian sojourn. Don Tomassino and Dr. Taza ply him with tales of the Honored Society during his long nights. Puzo conveniently introduces Filomena, the midwife who delivered Luca's illegitimate baby and who was then forced to throw it in the furnace, as a way of furthering Michael's education. First, Filomena's tale finally clues in Michael to a story that Hagent had told him he'd never hear until "you're a hundred." Second, Filomena expresses her gratitude to Don Corleone for interceding with Luca so she could escape to Sicily. FFC obviously was intent on treating Luca as a minor character, good mostly for the shock value of his death; and Michael's stay in Sicily only as a vehicle for meeting Apollonia and witnessing her assassination, not as a training camp for his future as a Mafia don.