You seem to have a real grasp on the context of the situation.She seems to be at the age where she is figuring out who she is as a girl flowering into a mental context of 'adult', and wants male attention.She would likely be doing alot of this behavior;out of adolescent stage curiosity,whether it were you or another male that brought so much enjoyment to her spirit.She enjoys you,your personality,obvious regard for her; which seems to go beyond selfishness,and your history together,which enhances and reinforces her drive to be flirtatious.There are obvious biological concerns for not becoming intimate with her.Even if it were ok with the two of you at her age of 18....somewhere down the line it would evolve to a circumstance that became troubling for both of you and likely end in a serious need for therapy.Your age difference...for the next 10 years (you are 20,she's 15....when she's twenty you'll be 25.....)5 to 7 years while being within this age range,will create so many differences between you (when you consider your own development at that age and how rapid and or transient it was),you wouldn't feel the same way over the course of time as either of you do now.The societal constraints on such a thing holds alot of stigma and judgement,and you couldn't avoid the reality of it.She has a 'crush' on you because of the history,her age,your accessibility,your obvious ability to care for her on a deep level......she seems to need that bond with you.I think you are great ,in that you have restraint,forethought,insight,and a genuine regard for her.I really think the ability for something like this to literally haunt you is an undeniable factor which pleads for perspective.I also think this could 'turn on a dime' into a very regrettable action with no 'edit/delete' button.Prior to age 18.If I were you,I'd seriously consider making an inner agreement to not find myself alone w/o the eyes of others....when I was with her.Her 'friendship',and the memories the two of you are making will sustain you both for decades to come.You are essentially her brother.And if you don't violate this very special 'trust' you've built with her,you will be giving her an irreplaceable gift.Would it help to train your mind to view her as your sister....would you draw a line deeper in that case?Anyway,as usual....I am replying far outside the 'norm'....that's just 'me'.As far as this 'I'm asleep' game....that's just sticking your finger in a flame til it burns.jmo