Literature and the family are intimately connected.
No doubt this is in part because the family is such a fertile source of narrative: stories are handed down from parents and grandparents to children and grandchildren; and also we tell ourselves and each other stories about the family in an effort, perhaps, to understand who we are and where we come from. These familial and familiar stories, for which we now draw on the narratives provided for us by psychoanalysis and culture at large, help to place us, to give us a sense of place. There's something reassuring even (or especially) about a story of troubled family origins. Perhaps it gives us a way out, but at the least a way to cope with and come to terms with ourselves.
There's also something about troubled families in particular that seems to demand a story. Perhaps it's the inherent interest of the "unhappy family," as Tolstoy's famous first line to Anna Karenina suggests. The family serves as something like a primordial site of speculation and gossip, as looks or words or anecdotes are exchanged about more distant relations: uncles and great-aunts, cousins and great-grandfathers.
But in the novel (at least) the family is not only context or source: it is also traditionally motivation or prospective future. Think of Austen's heroines and their search for a fit husband (and so another famous opening line, that of Pride and Prejudice). They're not just about coming to terms with origins, but also about leaving one family and starting up another, and the anxious moments of transition in between, the fear that in leaving the frying pan we are simply jumping into the fire.
So typically novels in end death, birth, or marriage: the end of a lineage, the beginning of another, or the establishment of a tie between lineages.
But the family is also a social metaphor for other sites within which identity is constructed, contested, or even broken down. For Latin America this is perhaps especially the case: Doris Sommer has famously argued that the region's foundational fictions are "family romances" in which the fate of the nation is allegorized through the vicissitudes of family plots. No wonder that this heritage should bear also on the literature of the twentieth century, that so many of the great books by which Latin America made its literary name should carry with them family trees, as though the novels themselves were merely commentary on such diagrams of familial interconnection.