This is a very touchy issue, to be certain. And, I think it might beneficial to see a counselor for the long-term, yes. In fact, it might be worth bringing your daughter in with you, if you can. This would help repair your relationship.
That said, I guess I'm having a bit of confusion. While I realize that they are both your children, your daughter has told you something that was tremendously damaging in her past. It was a behavior (by her brother) that is not only criminal, it's morally reprehensible. It's something that is certainly deserving of consequences. And, since those consequences cannot be legal (at this point, being that it was so long ago), shouldn't he at least be afforded the knowledge that what he did (and apparently continues to do to the family) is not to be tolerated? By continuing to have positive contact with him, you're sending the message that his behavior was okay. You're further sending the message to your daughter that you value your tenuous relationship with him over any empathy you might feel for her.
I know that this can be incredibly difficult for a mother - I've seen it more times than I'd like to admit (I'm a forensic psychologist, working with prisoners and their families mostly). But I'm absolutely sure that you could not be "responsible for his downfall." He, and he alone, is responsible for his choices. If you're taking responsibility for his current wellbeing, you'd have to take responsibility for all of it (including his bad actions, like the attempted rape). Clearly, you are not responsible for the latter, so stop putting the onus on yourself to be responsible for the former! He is an adult who is, and should be, responsible for his actions and their consequences. By caring more about him "falling apart," you're punishing your daughter for being the "together" one, which is potentially re-victimizing her.
I suggest you make things right with your daughter, empathize with her pain, and send the message to your son (once and for all), that you're supposed to protect people you love. Just being born into a family doesn't give you that privilege. You must earn your right to stay in everyone's good graces.