I wouldn't take it, for several reasons:
1. She's "about to be" a big-time lawyer? The law market is REALLY rough right now, and if she isn't even established yet then assuming she'll have the 50 grand to fork over in 5 years is a stretch. What happens if she loses her job and falls on tough times? Do you think she'd live up to it? Would you even want her to if it would put her in financial trouble?
2. It's a really unhealthy relationship to develop with your sister, being paid off for the decisions you would have made that she doesn't like or understand.
3. Even if you follow through on it, it could create distrust and awkwardness between the two of you. This is kind of an extension of #2, I guess, but it's already obvious that she has no trust for you and your decisions; do you really want her suspiciously keeping tabs on everything you do to see if you're living up to your pledge? The best way to kill honesty and trust is to involve lots of money.
I would tell her that what she's "offering" is a very controlling gesture and isn't helpful. You can't "fix" people by throwing money at them to do as you say. Whatever she thinks is wrong with your drug use, understanding and respecting each others' decisions and offering candid advice and criticism will do both of you more good than this bizarre arrangement of hers.