How old are you and the parties involved btw?
I can understand you feeling completely betrayed and for good reason. I hope you confronted her about all of this and got her response. If you feel her response was adequate enough to try forgiving her, by all means try. Explain the hurt it caused you and if she understands and feels terribly about what she has done it will perhaps make you feel a little better to have forgiven her to some degree. Of course, you should never forget what she has done and keep that in your mind for a long time, but be at least be as gentle as you can on her if she is truly upset and sorry for her actions.
When cocaine is involved, drinking, the party atmosphere, people are intoxicated etc.., it's easy for individuals to make mistakes-- even seemingly outrageous indiscretions. I've made my fair share in the past.
As far as you being out of the loop between your brother and the girl, try not to be any more intrusive than your brother wants you to be. Try to gain his trust and discuss a few things about why he should try to make good decisions in his life. He needs to develop a clear idea about some of the boundaries he cannot cross and how to deal with situations in which these temptations or risks come to light. Express your worries to him but most importantly show an interest in hearing his ideas about all of this and look to be more of a support for him, as in offering perspectives of your own for him to consider and not necessarily telling him what he must do.
Best of luck, do your best, and never blame yourself if you've been trying your best and something ends up happening. You cannot do everything, you're human, but I'm proud you're sincerely concerned and doing what you can.
