By way of explanation, after myself the brother in question is far and away the most liberal and libertine member of my immediate family. While he's probably similar to D in just not wanting to hear about it, I doubt the idea of our sister scoring some action disturbs him any more than it does me. What I believe does disturb him is the idea of that action coming from a personal friend.
To give a bit of background for context, I recently got the number of a girl I met through my friend who he's been jonesing for since the day he met her (I also think she's very attractive, but though we get along well enough in the limited interactions we have, we both realize our personalities are such if we ever spent any significant amount of time together sans the moderating presence of our friend or alcohol we'd kill each other). We've spoken at some length about her, and he's admitted several times to knowing full well he has no chance at anything more than friendship with her. Despite this, however, when he learned I had then used her number to contact her directly and invite her to the party last night, he got upset and asked me not to contact her myself anymore, explaining it would be similar to him contacting one of my ex-girlfriends without telling/clearing it with me. I then responded I honestly wouldn't care if he did that, as evidenced by the fact he did ask out an ex-girlfriend of mine in the past and it never bothered me in the least--to the contrary, I congratulated him and hoped things would work out for them where they hadn't with she and I.
At any rate, the girl never showed up, and my friend and I discussed the situation after he got to the party last night. We pretty much agreed he believes in and abides by an unwritten rule that, where romance is concerned, one's family and ex-(girl/boy)friends (and even friends or acquaintances one has a futile interest in) are off limits. From that perspective, if I or any other friend were to date or be in any way intimate with one of those people, he would feel that rule had been broken and his trust somehow betrayed (despite also admitting he'd be at a total loss to explain exactly how). In my experience, the great majority of people also seem to believe and abide by this rule.
Speaking for myself, on the other hand (and as I said above); in my mind there is no such rule. I wouldn't have a problem with any of my friends crossing that imaginary boundary--and I can say this with personal authority, having experienced 2/3 of the above in the past (friend with ex-girlfriend and friend hooking up with her before I could) and having come to terms with it and moved on with the friendship in both cases with little to no intervening drama. If it's my sister, she's a big girl. If it's an ex-girlfriend, that relationship is obviously over and there's no reason for my friend to deny his feelings out of some noble but misplaced sense of loyalty to me. If it's a girl I'm interested in romantically and my buddy asks her out before I do (or after I've been denied, or she asks him, or circumstances work out in any other way to the effect they hook up rather than me), not only is it at least half my own fault for not trying to make something happen sooner, it's once again a case of the girl being responsible enough to make her own decisions.
The thought suddenly occurs the reason I don't live by this rule is I not only acknowledge but wholly embrace the fact "it takes two to tango." When my friend says he would feel betrayed if I were to date a girl he like(s/d), at no point does any real consideration of her thoughts or feelings on the matter enter his mind. Never mind any possibility of her having thought it through or feeling the same way I do, all he sees is me "betraying his trust." Whereas when he asked out my ex-girlfriend all those years ago, I remember thinking she must see something she likes in him, and especially now we were no longer together it was in no way my place to try to have any kind of say in who she (c/sh)ould and (c/sh)ouldn't date.
Going back to my brother, I'd suspected the motivation for his asking me that question was learning of this conversation I'd had with my friend, and though I doubt I'll ask him about it, I'm confident now my suspicion was correct. The disconnect between us seems to have been the fact I focused more on my sister's part in the situation while his focus was entirely upon the friend.
