The article's first paragraph has some alternative factors in selection
> Traditional evolutionary hypotheses, such as those invoking kin selection, mutation-selection balance, and evolutionary mismatch don’t quite explain this.
There's lots of explanations here but one that is often mentioned is "kin selection". If you increase evolutionary fitness of nephews, nieces, etc. There's many others, some of which apply here and others which don't.
> Traditional evolutionary hypotheses, such as those invoking kin selection, mutation-selection balance, and evolutionary mismatch don’t quite explain this.
There's lots of explanations here but one that is often mentioned is "kin selection". If you increase evolutionary fitness of nephews, nieces, etc. There's many others, some of which apply here and others which don't.