[E]mpirical work in monogamous societies indicates that higher degrees of relatedness among household members are associated with lower rates of abuse, neglect and homicide. Living in the same household with genetically unrelated adults is the single biggest risk factor for abuse, neglect and homicide of children. Stepmothers are 2.4 times more likely to kill their stepchildren than birth mothers, and children living with an unrelated parent are between 15 and 77 times more likely to die
"accidentally." . . .
[D]ifferent household compositions impact cortisol levels (a stress hormone) in children. Children in nuclear families with only genetic parents showed the lowest cortisol levels. By contrast, children in households with distant relatives, stepfathers and half-siblings showed the highest cortisol levels of any household composite in the sample[.]
It also strongly implies, without expressly saying so, that intrafamily strife is much greater in non-sororal polygynous families (i.e. families where men have multiple, unrelated wives), than in sororal polygynous families (i.e. families where men have multiple wives who are sisters of each other), and that this is the case in large part because children in sororal polygynous families are much more closely related to each other than other half-siblings.