I’m interested to know how the woman feels about doing it with her brother’s best friend?
Watch Scarface with your roomie…oh, wait. Bad idea!
In your 30’s and still have a room mate, woulda thought the sex type thing to be worked out by now. Just don’t brag how good (or bad!) she was. That is rule #1 after you break the other rule #1.
SHAKES, Jimmy and Stonebow are the only people making sense. One of the first rules of The Guy’s Code is to not sleep with your friends sister. Regardless of who innitiated it, no guy likes to think of his sister as one of his friends sexual conquests.
There is also the issue of if one person wants more out of the relationship than the other does. It does not help your relationship with your friend to have his sister bitching about why you won’t call or to have 10 pathetic messages from you on her answering machine. It makes everyone uncomfortible.
Now everyone else seems to want to give you PC advice that sounds good on paper - you are both adults, he should not get involved, blah blah. I am here to tell you that The Guy Code has been created over the centuries for a reason that goes beyond mere male postering.
To this day, my friend (let’s call him “Carl”) is scared to death that I will reveal to our other friend (let’s call him “Lenny”) that Lenny’s sister blew him one night at a party eight years ago. It didn’t help that for years Lenny was like “my sisters too good for any of you clowns”, “if you guys ever touched my sister I kill allsya” and so on meanwhile I can’t keep a straight face.
I wish my sister would date my best friend. He’d be such a huge improvement over the losers she usually dates, I couldn’t possibly complain.
I know the Code, but believe me, if you met my sister’s last boyfriend - known in my family simply as “The Pizza Boy” - you would understand. I’m willing to make an exception.
Ok… Yes, it IS a double-standard, No there is no real logical reason for this (although I will endeavor to provide one), and Yes it may be silly… BUT bumpin’ uglies with yer best friend’s sister is a big No-No! Now, dating and relationship sex get a pass here… but pick-up or casual sex is [italics]Verboten[/italics]… don’t blame me, I didn’t make up the rules I just enforce them.
Is it wrong to sleep with your best friend’s sister? Not necessarily.
Is it wrong to be inconsiderate of your best friend’s feelings? Definitely.
I’ve lived in a house with a guy who was sleeping with my sister. It wasn’t a big deal, even when the other housemates tried to make it a big deal. But I can’t condemn your friend without knowing: Is there any reason he might be upset about this other than just being a prude? For instance, if you think of women as “conquests” then I could see your friend not wanting his sister to be a notch on your belt. Or if you’re HIV positive I could see him being a bit upset. Or if his sister was previously a virgin, I could see him feeling extra protective. I’m not saying any of these scenarios are true, I’m just pointing out that it’s impossible to form a real opinion without knowing exactly why he’s upset.
I’d also like to know what the sister thinks about all this. And what did she tell her brother?
Yep, rules to live by.
Yea, I realize I should join in the “people can do whatever they want” thing, but frankly, you don’t mess with friend’s sisters. You’re just breaking the Guy Rules.
Yeah & if you split up, you have to choose betwixt your sister & your best friend sometimes. e.g. Who would you invite to your wedding?
My best friend was my best man at my wedding. Mrs. CaptMurdock’s bridesmaids include one of my ex-girlfriends. No joke.
My sister was also at my wedding…but only because I could not have my neice as one of the bridesmaids and not have my sister attend.
In essence, if my best friend slept with my sister, I would severely remonstrate him…“Oh, c’mon! You can do better than her!”
Believe me, my family puts the “funk” in “dysfunctional.” 
Well here is my opinion…It may not have been wrong…but it was in bad taste, IMHO. I mean you can, in theory, “f” whoever you want to, and since it was concentual… however, my guess is you knew that when your friend found out that he would be upset with you. And because you are the friend, and not the family, you are going to be the one that he is more pissed at, even if she did initiate the “interaction”. Bottom line, I would say if you care about this friend and his feelings, then choose out of the many other women out there to “f” and just stay friends with his sis. 
Boy, am I glad I don’t have an older brother. Sex is complicated enough without these issues.
The only way I could contemplate being upset at a friend boiking my younger sister would be if I knew they had different expectations about the aftermath. Then again, I know my guy friends are all decent human beings, so I can’t see this being an issue. (That, and my sister and I are so different from each other that I honestly can’t contemplate any of my guy friends wanting to boink her, and vice versa.)
Actually, I’d be so happy that my sister was boinking someone other than the lying, manipulative, cheating sleazeball she’s been boinking for the past 7 years that I’d probably shower the guy with gifts.
I did it. My best (male) friend in high school had a twin sister who was hot and sleazy. I went over to his house one afternoon, he turned out not to be home, she and I ended up doing it on the hallway floor.
It was fun, but he was really weirded out by it. He got over it pretty quickly, but the deal was that he basically hated his sister and couldn’t understand why someone, particularly someone he related to and thought he understood, would want to get naked and sweaty with her.
Having four sisters of my own, I can understand that question.
The “little sister bond” part of my brain doesn’t respond well to sweeping social generalizations. Anybody can have sex with anybody they want, including my sister. That doesn’t mean I’m going to shake hands with the guy that sleeps with her for kicks.
There’s a reason for that. I don’t buy Maxim and I’ve never seen the Man Show, but those things pander to the “typical guy.” The typical guy would hate you for sleeping with his sister. It’s a Neanderthal, alligator brain kind of a response, but any guy is going to react the same way. Again, I’m not defending the injustice of it all. I’m saying it’s natural and you have to expect it to happen.
Modern guy rules say you can have sex with your friend’s sister. But only in a serious relationship; no casual sex.
No change on the exes rules; never is still the law.