You know, I'm that way with some of my closer guy pals sisters. It's almost as if they're MY sisters, and if I hear anyone knocking them down or insulting them, or on the other hand, being crass, I get protective.
My situation is a tad bit different in that my sister is 10 years older than me, so I never really got that "big brother's gotta protect" kind of feeling. The first guy was a jerk and a bit of an ass, I was too young to know or understand. Except for her past boyfriends being huge dorks and sort of losers, they've been good enough guys, for the most part, so no real worries there.
I did, however, warn my sister's husband once, just a blanket warning, as they began dating, that if her ever tried to physically or emotionally abuse her, he'd answer to me.
Most of my friends are solid, good guys, so It wouldn't be a concern over potential abuse, because they don't do that. My friends aren't cheaters. They have jobs or are at least hard working. They're great guys.
1) Guys will have a few friends that are good guys, but some guys will be horndogs.
2) If there's a breakup, then you might have to take sides. You believe the friend, and your sister feels her brother always takes other people's sides. If you believe the sister your friend may feel like you don't trust them. You do choose a side, the other can feel rejected. It's not a win win situation.
3) As in #2, now if there's accusation of abuse or worse, then you lost a person you thought was a friend.
4) Sometimes people like "clean" breakups. The best thing is that I don't deal with my first girlfriend. I don't have any of that. "Out of sight, out of mind."
5) We're protective in general of our families.
6) Sometimes we guys like separation. It's like working with our wives. A lot of times, life is more tricky, because we now run the risk of our personal stresses inhibiting our work space, and vice versa.
7) We don't like hearing about friend's sexual conquests, and if we know the past sexual conquests and stories of our friends, we don't want sister's being talked about like that.
8) If there's a temporary argument, you have to watch how much you/they come over. They come over, and you want to hang out, but they want relationship time with the sister; or, they're trying to see you, but your sister wants to spend time with her boyfriend. That's just great. Either way, now you've got a potentially awkward situation.