It certainly wasn't normal or accepted, as far as I recall (I am in my early 40s). However, date rape seems a more widely comprehended concept than when I was a teen...maybe kids better understand the dangers of partying too drunk/doped up, but I'm not sure.

Personally, I have never seen sex with someone who is drunk/stoned into oblivion as an opportunity. (Not unless it was preceded with the statement "Let's get drunk and have sex!!" )

If your brother said he was going to "take care" of her and then took advantage, I think that was a pretty lousy thing to do, even if the girl was being an idiot. What happened, and whether rape actually occurred, may only be known between him and the girl...or maybe even only to him. But it doesn't sound unreasonable that you trusted him.

I don't think you need to beat yourself up over this, but it's healthy to question it. Have you ever talked to your brother about it?