Is there any strong evidence that having a sibling makes a human any less likely to survive to reproductive age? I believe (without evidence) that we could just as easily hypothesize the opposite: once siblings are mature enough, they can help and defend each other.
Further, doesn't this go against the idea that siblings are altruistic towards each other because those who are will be more likely to continue their (family's) genes? (Swap order of cause and effect there for more correct evolutionary thinking, of course.)
Have evolutionists worked out which is more fundamental, sibling rivalry or sibling altruism?
In any case (but particularly without being clear on the answers to the above), like many evolutionary arguments this seems like a little bit of a just-so story.