Getting out of a sticky situation
So the other day, my dad told my sister he would give her gas money. The next morning, he had a doctors appointment and forgot. my sister borrowed the money from me to fill up gas in her car, saying she would ask my dad to repay me and telling me not to tell dad and that she would take care of it ( previously he told her not to borrow money from me). She hasn't discussed it with him as of a week later, I reminded her today and she said she would do it but threatened that if I remind her, she would tell my dad I tried to buy cigarettes using someone older than me, which is not true and I have never done a drug in my life. But my dad would still believe her. I think she dose not plan on paying me back and is using this as leverage against me.. If I tell my dad that she borrowed money from me, he will get mad at her because she was told not to borrow money from me, and then she will tell him I tried to buy something illegal ( a lie and he would still believe her). I do have leverage on her for doing things she shouldnt be with evidence.
I don't quite know how to go about this situation, will somebody please tell what they would do or give me advice below?