I think it's a waste of time, personally. I don't think it will make a dime's worth of difference in his lifestyle decisions.
"No" is a complete sentence. If you want to be more polite, then "Sorry, no." If you want him to stop asking you, do what your sister did (minus giving him the money first). "Sorry, I can't help you. Please don't ask me again."
I totally get your frustration. But the finer points of "enabling" will be lost on him.