Because she tried drugs, and got to the point where she relied on them and became addicted to them.
She could have said no to them, but didnt. I doubt the first time she ever tried them, she had an uncontrollable urge to do it, Im sure she tried it out of curiosity. I have a very close friend whos addicted to drugs, and he is 100% capable of admitting its his own fault, and that he allowed it to happen to himself. There are those who can admit they are in the wrong, and those who always try to find something to blame, to take responsibility from themselves.
I hope your sister can find strength and overcome her addiction.