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What makes a brother protective over his sister even when they're the same exact age?

Generally loving siblings have each other's back but I've noticed this deeper protective stance (different from 2 brothers and 2 sisters) whenever it's a brother towards his sister.

Even if he were younger than her or the same exact age, he still wants to protect her. In a couple sad cases, he went as far as putting himself in danger to save her; getting injured or killed in the process but the sister was saved. No one ever taught him to really put his sister's safety first. He just does it so automatically.

My friend's brother is her twin. They're both exactly the same age but he's still very protective towards her and would do the same too. He won't let her (risk her life for him) do the same for him. Why? What makes a brother very protective for his sister even if it's her twin?
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+1 y
It seems they learned that from an early age. For instance this kid was only 6 years old when he got in between an aggressive dog charging at his younger sister. At such young age, he already put himself in danger.
https://www.youtube.com/watch?v=ELgFpo1kWsg
What makes a brother protective over his sister even when they're the same exact age?
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