Only plans to kill my sister's addiction that account for my sister's feelings will work.

False. I assume that plans like "kidnapping her and keeping her in a private prison without access to heroin for a few months" would also work. Illegal and unethical perhaps, but still technically possible.

But I guess in real life it means something like "if she will not like the approach, she will sabotage it", which is probably true. :(

Only my sister can fully account for my sister's feelings.

Other can still make a guess, and maybe guess incorrectly, and maybe guess correctly.