i know how it must be frustrating but look at things from her point of view. she has spent 3 years, possibly working hard for what appears to be nothing at all because she has no degree. she must feel like somewhat of a faliure. add on top of that a load of debt. i bet she feels like she's wasted her life and has got herself trapped in a situation she cannot see an end too. people plan out their lives with high, sometimes unrealisitic, expectations of what they will be like. to have to confront the reality must be devastating. honestly, i feel sorry for your sister more than i feel sorry for you. things must have affected her badly if she self-harmed and is depressed. no doubt, her life feel like it's not worth living right now and she feels worthless. is is any wonder she has no movtivation when she has spent 3 years working at a degree to come out with nothing at all? she self-esteem and confidence has taken a hard bashing.
if you have a good relationship with your sister may try and get her to open up with you. if you dont then try and make conversation with her about anything but her uni/debts/jobs etc. over time she may feel like she can be comfortable aroudn you and may want to talk about her problems then.
there is not a hell of a lot you can do to make her "grow up and smell the coffee". nagging or even hinting that she is lazing about the house and needs to get a job will probably just make her sink further into depression. like i say, she probably feels like a faliure as it is. i think in time she will learn to come to terms with her failed degree on her own or maybe, if you talk to her about it (if you have a close reationship).
and she will probably never continue to sponge off your parents. can you honestly see this continuing for many years to come? i dont know about your family but i doubt this is likely.
also, what are your parents doing in this situation?