I have been where she is. My father bailed me out several times. Only after he died did I learn to stand on my own two feet and take full responsibility for my debts.
In short, there's nothing you can do for your sister until she actually asks for help, and if that means she ends up going bankrupt, then that's what it'll have to take, I'm afraid. All you can do is be there for her when it happens, and let her know that just because she's a poor financial manager, it doesn't make her any less of a person.
Do not, under any circumstances, give/lend her money. Food, a few items of clothing etc etc, fine - but no money.