I think my sister is taking drugs, what should I do?

Hey, as the title says, I think my sister is (repeatedly) taking drugs. A little backstory: My sister is 20, and we both grew up in a less than ideal household. My father is a narcissist and often shouted at us, etc. He didn't hit me, but I think he hit her, but it all points more to mental abuse. Since she's my half-sister, she doesn't have to see him anymore after the divorce. After the divorce, we also found out that she was taking drugs; there was something like that at her school (Adolpfinum in Moers), but then I think she got away from it.

Now, after three years, my sister is mentally deteriorating again and I'm pretty sure she's taking drugs.

1. .her room always smells strange, like something sweet or incense. She always smells like that too.
2. Lately, her eyes have been watering all the time, which indicates that she's smoking something (I've already read something about this on Google)
3. When I came into her room, there was a small towel in the shower with two lighters on it and black powder was scattered everywhere.
4. She seems a bit "sluggish" lately, but I think she's trying to cover it up.

By the way, my sister is autistic. I'm hoping someone here can help me and tell me how to approach her or something.