What if I tell my sister to stop talking to me until she at least tries to get her shit together?
What makes you want to stop talking to her?
Everyone knows she's on the streets doing hardcore drugs
I can understand why you want to take extreme action. What she needs is for someone to throw her a life preserver. Maybe you could conduct an intervention?
I believe in tough love, not life preservers. It's a sink-or-swim world out there.
If she has substance abuse issues, she's probably already feeling alienated from everyone. The last thing she needs is a close family member to rub her face in it. Try to muster a little empathy and guide her off the streets and into rehab.