In most Darwinian models there is competition across siblings for resources and parental attention, from the womb but also stretching into adulthood. Siblings who do well therefore will be hyper-aware of the strategies employed by their brothers and sisters. They will need to counter those strategies on a very regular basis and furthermore they will on average be deploying similar strategies themselves.
At the same time, siblings probably won’t see each other as so evil by nature. They will be realistic about motives — some would say cynical — while at the same time recognizing that the siblings are probably, on average, no worse than themselves. Plus there is a natural genetic and also family affinity.
How about mothers? Genetically speaking, mothers often adopt the interests of the sibling as “their own.” For instance a lot of mothers died in childbirth before modern medicine, when alternative biological arrangements would have given the mothers greater protection. So the children can commandeer the loyalty of the mother (and sometimes the father) more readily than they can commandeer the loyalties of their siblings.
Mothers are therefore often deceived about or simply tolerant of the manipulations employed by their children on them. In other words, mothers worry less about moral hazard problems with respect to their children. The siblings will in some respects understand these strategies better than the mother will.
The other children may feel that a mother should punish (or possibly but less likely reward) the other siblings more. And “Johnny is being a stinker” will be a more frequent complaint than “Johnny is possessed with Original Sin.”
In turn, mothers may worry more about problems of type. If a mother is hyper-aware of the faults of her children, she may do a better job of protecting them or teaching them how to overcome those limitations.
A world where fewer people have siblings may be a world where recognizing moral hazard problems may be for many people less intuitive. Is it also possible that men may on average be more aware of moral hazard problems than are women? And women more aware of problems of type?