Dude, There is three possibilities for your problem if you tell her:- First is 1. You might tell her and she can be amazed and probably say that she will think about it. And hopefully you guys will be together. Second is 2. She calls you DAI, so maybe she really think you as a DAI. And when you tell her about your love. She might throw a Fit and she will be like "What I call you Brother and you act like a arse." That's it I am not going to talk to you ever again. Last but not the Least 3. Call her boyfriend or real brother and bit the crap out of you, till you remember to call her sister again. So what really happened---update us please.