I feel like I’m lying to my boyfriend.

A little backstory: we have been together for a year and a half. His sister has just started opening up to me. We have been hanging out more regularly. I am not trying to become her best friend but I am trying to maybe be a positive influence on her, she’s kind of lost.

Anyway, she opened up to me that she does drugs. (Cocaine) She said she doesn’t want her brother to know. I am happy that she trusts me. She has BPD and is bipolar so I’m nervous about upsetting her because she has a tendency to freak out. But I feel like I’m lying to him because it’s a secret and I don’t know what to do.

I want to keep her trust but I also don’t want to lie. He has suspicions that she has done it or is. Do I tell him? I’ve known for a few days now. Does that make it a lie? She told me in confidence, I don’t know.