OP, DS is 28. You are not responsible for his actions and I don't think you need to discuss them with him. You say it is sad you don't see him on his own: I wonder if he feels the same about only seeing you with DH. Could you see him just you and him occasionally?
DD is the big concern here. Cocaine is not my thing but the stand out piece of information for me is that DD felt uncomfortable but didn't do anything to alleviate her situation. There could be lots of reasons for this: the priority for me would be to try to equip her to advocate for herself in future.
When my friends lived in flats and we would all pile back after a night out I slept in random lounges with friends of the host in sofas without feeling uncomfortable in any way. At no point did I feel I was going to be attacked by someone the host knew.
If I had, I would have found another way home. Also, asking to sleep in her brother's room with him and his girlfriend is not a usual request. She sounds like she hasn't got her head around how these things work. She might be the sort who needs to sleep in her own bed with everything arranged how she wants it, which is fine, but in that case she needs to go home rather than expecting the world to continuously adapt to her wishes.
She was 21, had drunk herself and might not have been thinking especially clearly but if she was so repulsed by her brother being sick on the way home, which is fair enough, she should have been looking for a way home.
Is your DH your DS's father? It just feels like he might not be.
Presumably she has learned from this and will not be going on nights out with her brother in future. I feel it would be unacceptable to do that again and complain. His sort of fun is not hers: she needs to not do that again.
I would rather keep out of this, OP. I suspect you getting involved won't help and they are both adults, even if DD is a very young 21. Your days of refereeing sibling disagreements are long gone.
I do wonder if DS might have a different take on the situation and I'm not sure why I wonder that. Might the truth lie between their different perspectives? In my world, going back to someone's flat after a night out and telling the homeowners who should sleep where isn't quite cricket either and won't make DD's social life go with a swing.