Ask Amy: Protective brother wakes up to a terrible surprise

Dear Amy: My younger sister and I are young adults. We are extremely close, and have many mutual friends. She is educated, attractive, successful, single and often regarded as the type to take no nonsense from guys. I am sometimes regarded as the slightly overprotective older brother.
This past weekend we took a trip together, went out to bars and split a hotel room.
I allowed a mutual friend to stay with us, because he had been drinking. This friend is known for his exploits with women.
I fell