My fiancée’s sister (18f) tried to sleep with me(25M)

For context; I basically became a “father figure”‘for her. She moved in with us. Last night she tried to sleep with me by telling me that she likes me a lot and that whether I wanted to be with her in her room. I,e sex. I asked her what she meant by that, and she said sex. Of course I turned her down.

This all happened while my fiancee was gone. This is obviously a trauma response; so how do I even begin to tackle this? She has always been like a friend to me and nothing more. Not sure what to even do considering she’s living with us.