What makes a brother protective even if the sister is his same age or older?

My brother is my fraternal twin and 3 years ago (we were then 14 year-olds), he put himself at risk getting me out safely. He didn't want me to stay with him. He got rescued afterwards and only had minor scrapes from small falling debris, nothing major.

There was a chance he could've gotten injured badly or die if rescue didn't arrive on time. This has been a known behavior for ages; a brother protecting his sister even if it means getting risking his own life or safety. What makes a brother protective (not focusing on his own safety) even if we're his same age or older?

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He preferred my safety over his. Why? What's the cause of his known behavior?
What makes a brother protective even if the sister is his same age or older?
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