Even enablers need to hit bottom, easeful, no matter what their age.
So your husband would continue to enable the addict, pushing her deeper into her addiction, maybe pushing her closer to death, in order to spare the other one? Would you feel good about participating in this in the long term? If you look back ten years from now, would you feel good about your actions?
What about telling the enabler that you're doing what you can, and then doing nothing at all (rather than very little)?
Better yet, what about letting your husband worry about this complicated mess that YOU DID NOT CREATE. You didn't cause it, you can't cure it, you can't control it. All you can do is get plunked right in the middle by the involved parties if you let them....and since you asked...I don't think that's at all fair to you.
As pjbs said, consider turning this one over to your HP. This is not your battle.
My two cents, all polished up for you. Hugs and strength to do what's right for YOU and not somebody else.