My sister must be is desperate!?

Don't be a co-dependent. I'm sure you know what this term means if you have an addict in the family. Let your sister make her own mistakes-most people don't learn unless they go through it. Don't sit with her while she cries about what an her boyfriend is, don't loan her money, don't let her stay at your house when they fight; let her know if she goes through with this that you'll still love her but you absolutely refuse to have any part of her relationship. Once she's out on her own with no support system, she's likely to hit rock bottom and to realize that she's going in the wrong direction.