4 ideas
Old Fouan, a Beauce peasant too old to farm, divides his land among his three children, Fanny, Hyacinthe ('Jésus-Christ') and Buteau, in exchange for a promised annual pension and upkeep. The children haggle over the payments, pay late or not at all, and pass him between their households as a burden.
An elder who hands over property in return for promised care gives up the only thing that made the promise enforceable. Once the land has changed hands, the heirs' incentive flips from courting the giver to minimizing what he costs them, and the pension becomes a debt they resent. Care secured by a contract with no retained stake decays into neglect.
Under the Napoleonic Code every child takes an equal share, so each generation cuts the family holding into smaller parcels, which Fouan's children assign by drawing lots. Each heir fights for every furrow, while the pieces become too small and scattered to farm efficiently. A rule meant to be fair between siblings turns them into rivals and impoverishes the land itself.
In the novel, marriage, sex, childbearing and murder are each decided by what they do to a claim on land. Buteau and Lise assault and kill Lise's sister Françoise to stop her share passing to her husband Jean Macquart, and because Françoise leaves no will, the land stays with the blood family while Jean, the outsider, ends with nothing. Read this way, the family is a mechanism for holding and passing on property, and affection lasts only as long as it serves that purpose.