What can I do about my friend?

I have this friend and she is very promiscuous. She is only 16.Which is fine because that's what she chooses to do. The thing is though that she texts my 18 year old brother. My brother doesn't like her at all and she's just embarring herself and me as well. He also tells my parents some of the things she says. I believe if she keeps doing this it will end our friendship. Not by my parents force but by my choice. It makes me feel used. I don't want to say anything to offend her. I've hinted at it before and she stopped for awhile. But last night she asked him for a ride in the middle of the night. What do you think I should do? Thanks.