You are the adult here and you really, really, really need to set some boundaries both physically and emotionally. She needs to understand that her behavior is not really appropriate. I would worry that she would have poor boundaries with other people if she doesn't start learning them with you, someone who loves and cares about her.

Until those boundaries are clearer, limit your time together to times when you are around other people. That will help establish an appropriateness in your interactions with each other that she just needs to start learning. An intimate relationship between you two is really NOT okay; it would be incestuous and could have serious long-term repercussions for each of you. But a loving, supporting, caring, healthy family/friend type relationship would be entirely appropriate.

Isn't she 15? She is old enough to understand this intellectually. It won't be an easy thing for her to hear or accept emotionally, but she will have to learn what is okay and what is not okay. That's just part of growing up.