"She's my sister!"

So far in this blog, I have failed to mention the role of the male counterpart of the subject of my blog.

"She's my sister!"

This sentence, I once thought, was something I would only hear on a show like 'Friends' when Ross found out that Chandler was having sex with his sister Monica. However, it has recently come to my attention that I am, as always, greatly mistaken.

Recently I've learned two new lessons in life. One: I goes unsaid that my male friends are not supposed to see pictures of my sister.
One Monday morning, I was standing with a group of friends (of both sexes) just talking. A guy decides to take out pictures of family members and show them to us, one of which was a picture of his little sister. I assumed that the pictures would go around us all and held my hand out to the person next to me. Suddenly the owner of the pictures tell me: "You can't, dude! She's my sister!" Till now I still don't understand why. It can't be because of the only reason I can think of because she's way too young (around 13), and I just can't find an explanation for it. So I'm left with this life lesson: Be unwilling to show guys pictures of your sister.

The second lesson was a result of the following event:
A friend of mine was telling me about his little sister who happened to go to the same high school as I did last year. Later that day I met a cousin who is also a student at the same school, and asked conversationally: "Is there a girl named so-and-so in your class?" after which he said "No."
A few days later I told the friend that no one seems to know his sister there. And the would not be lying if I said that I was absolutely amazed by the reaction. He said angrily: "How could you ask about my sister like that? She's my sister for God's sakes!"
So lesson number two: Never mention a friend's sister to anyone else other than the brother.

Will someone please objectively explain those two events for me. Were those reactions reasonable? I don't know how a brother would feel in a situation like that because I only have brothers, so I can't judge. So if you can help me with that part, please do. Because if they were just being a little overprotective, I'd like to know. I don't want to draw conclusions about people before knowing the whole story.

I know this much though:
I as a male have the advantage of being able to engage in any sort of sexual activity because there is no medical method to determine if I actually did or not. This results in the fact that when a guy has sex before marriage, it's called "youth", but when a girl does it, she's a whore (I do not condone either case, so in my mind both are whores).
I don't think that the save "advantage" as men, but that men be stripped of it altogether, in order to make it equal. If a man is to demand a "pure" wife, then I think it's only fair that he be the same. Or am I mistaken.