Guys, Why do brothers act this way when they see their sister talking to a guy?

So I was talking to two guys and all of a sudden my brother and his group of friends ( all guys) walks past us. My brother and his friends kind of knew the two guys I was talking to. My brother looked shocked lool and my brothers friends were saying to my brother "look, look over there" while smiling. Also my brothers friends were staring at us the whole time. Why do guys act like this haha. At the end it was pretty awkward situation especially to the guys I was talking to.
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Guys, Why do brothers act this way when they see their sister talking to a guy?
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