my sister is inhaling computer duster

I can’t share the whole story and not even sure if it’s the right thing to ask the internet… my elder sister got addicted to smelling nail polish remover and computer duster. I first noticed her doing it years ago in the bathroom, she denied everything. This whole summer she’s being doing it in our room in plain sight together with her schoolmate. I’ve tried to tell mum but my sister turned it all into a joke. She doesn’t smoke and has always been prim and proper… so mum didn’t take it seriously. She never saw her huff that stuff and she was never home during those moment when my sister acted weirdly. She works till late night and is practically never at home.

I don’t even know if this can become an addiction? Can someone give me an insight as to how I can stop my sister from doing it? She believes this is harmless, not like weed or drugs.