You're right. Your sis is your sis no matter what age.
But!
Once my sister was over 18 I reckoned she was old enough to make her own decisions on most things. If she wants to go get hammered then lock herself into a room with some horny dude that she's already told she wants to make out then what right do I have to get pissed if it happens? I wouldn't have appreciated her interference when I was trying to get laid ~ I expect she'd prefer if I extended her the same courtesy. If my sister had ever hooked up with one of my friends I wouldn't have give a shit...so long as I didn't have to see it, hear it, or clean up after it. I'm 4 years younger than my sis so the odds of that ever happening were slim to nil but still...
Good Lord....when the sis grows up and gets married or is in a years-long dedicated relationship you gonna get mad if she's getting laid while drunk? I'm not saying that there's nothing to get riled up about in CrownVic's sister's situation...but knowing only minimal details leads me to prefer not to immediately jump on the bandwagon and believe that CrownVic's friend is completely in the wrong (Remember: CrownVic was the one willing to leave his drunk sister there unattended in the first freggin' place!)
If the sister is a minor, however, all bets are off.