Have you tried to seriously talk to you sister about your concerns?
In your post you said when you talked it was kind of like talking to an alien......where the two of you close before and now drugs are in the way. Your concerns sound very valid and maybe you should tell her and give examples as to why you are concerned, without judgement.
It takes alot for children services or the police to get involved.....they have their hands full.
Maybe you should reach out to her....