Today i went over to a friend's place just to pick some stuff up (some software and other crap). I don't normally go over his house too often although when i went over today his sister was home as well.
His sister is a bit older than me (22 i think) but really good looking. She's pretty smart and also very friendly as well. Along with talking to him a lot of my attention was directed at her, of course

, and she also seemed to keep coming into his room to talk about random shit, so then i could say something to her as well and we both smiled at each other and made eye contact.
After a bit i went into the kitchen just to fetch myself a drink (i asked and he said just to grab one in the kitchen, i am not a rude bastard

). She heard me fumbling around a bit so she came in and helped me out by gettin all the stuff for me and serving it up. I was just asking about her plans for tonight etc and some other random drivel and questions which i am quite good at and my friend came looking for me cause i'd been gone for 5 minutes or so.
I could tell he wasn't happy with me, cause i imagined he was defensive about his sister and annoyed at the flirting etc i was doing. This was more evident when he told me she had a boyfriend, which was out of nowhere. I don't really have much of an idea if she was interested in me or not, or just being overly friendly, but i guess that doens't matter too much at this point.
Anyway, with that little story, i was wondering if there's an unspoken rule amogst some guys about sisters being off limits? Is anyone dating a friends sister, any awkwardness?
I have a sister myself, but she is much older than me and none of my friends would have a chance of going out with her anyway, so i've never really been in a defensive position of my dodgy friends trying to crack onto my sister. Although i imagine it's annoying, and maybe even worse to think that one of your friends may be sleeping with your sister?
Always seems to be a very delicate balance....