My Gracious!
Isn't it sad when you have tried everything, and it seems all for nothing? It's possible that it's just too late for your sister. But, as far as the children, do what you can to get custody (or just get them away from her). Get them now while they are young (i'm assuming they are young), and get them away from the obvious dysfunction that surrounds them in that house. It's obvious that she is a horrible mother as long as she answers the call of the drugs instead of her kids (leaving them for 3-4 days--NO EXCUSE!!!!
).
As has been said, she's made her choice. She'd rather be addicted to drugs than a good, decent mother. Time to take a stand.
All the best to you!