What a horrible thing addiction. I can understand and relate to your anger and frustration. I can also understand your concerns. It is one thing to let the addict and their enablers fall, if fact it is the right thing, but a 10 YO child is another matter and without some intervention she will follow the same pattern. She is learning how to manipulate and lie with no consequences. Your parents may be doing what they feel is best, but it didn't work for L or K, so why do they feel it will be any better or turn out any differently for the child. You know the saying "insanity=doing the same thing over and over, expecting a different result"
No child should be subjected to the whims of people that do not put their welfare first, be it the addict or others turning a blind eye to the consequences.
I may not have said this exactly right and I have no answers. but do whatever is necessary to save this child, she is the important, innocent one in this.
Blessings,