Show her compassion, figure out what she wants to do, then help her get there. Why is she like this? What motivates her in the morning to get out of bed? Start there.
My best friend (who I consider my brother) has a similar story, he dropped out of college, got into some trouble, and worked a series of temping jobs while he took technical classes at the community college. He lived with his folks for awhile, bounced between some crappy girlfriends and flirted with depression between hangovers while doing menial jobs and reading existential philosophy. He lived with us in Anchorage for awhile while trying to get back on his feet. He stayed with us for a year (for free), then went back to Oregon.
The whole time he was with us, we talked about his future and what he wanted out of life. He figured out what he wanted and now he's doing it. He lived with us while I worked an insanely time consuming job, a new baby, a pregnant wife, going from paycheck to paycheck with minimal money in the bank. Why did I help him out? Because he's my friend - and I'd do it again in a heartbeat. Today he's back in college, he's learning computer science (and doing well) and living with his folks, in another year, he'll enter society as a productive member, and will have a job skill. If he falls on hard times, I'll help him again. No questions asked.
Help people out, it's that simple. Give them the tools they need to succeed at what they want to do and they likely will, but they've gotta know what they want to do first. If it were me, I'd try to find out what motivates your sis, then go from there. Cracking the whip on her ain't going to work if she can retreat back to mama - and likely won't actually solve the problem. Being a hardass only works if the person believes that you have some authority over them - you're better off trying to motivate and encourage her to do something that she wants to do.
