She calls me her "little brother"

Ok, so almost a year ago (this September it will be a year), I met this Japanese girl at the university I am going to. She is from Japan, but is studying abroad here, and getting her degree here in America. It is kind of an odd situation how I met her. I saw her several times sitting alone at college. She was really shy, and has a thick accent. Some people would talk to her, but would get frustrated and say rude things to her about her accent. Other girls would talk behind her back about "her style." I go to a really cliquey southern university. I felt bad for her, so one day I introduced myself to her, and would talked to her. Yes, she does have a very thick accent, but I am patient with her. After a few weeks of talking to her, I introduced her to my family, and included her on family gatherings, and took her to sights around the area. I was doing this as a friend, because I felt bad for her, and I didn't want her experience in America to be terrible. After a few months she really started to warm up to me, and would always be near me. I am not joking, she would literally follow me everywhere, even when I went grocery shopping and stuff like that. After spending so much time with her, we grew a lot closer, and we started dating. Now 3 months ago, she asked me how old I was, and I said I am 21. It is like I said a magic word, she smiled and said she is 22, she thought I was older than her. She said something like: "You are my little brother, I always wanted a little brother." Now she hardly calls me anything else but "her little brother." When she is talking to her friends online through a webcam she introduces me as "her little brother" and would say "isn't my little brother cute?" I asked her why she calls me that, and she said something about how "we are no longer friends, we are like a family, and you are my little brother, so it is my job as your big sister to love and take care of you." Even since then she has opened up even more, and spends a lot of time at my apartment, almost every day she makes me all of my meals (Japanese foods of course), and is constantly making me Japanese desserts. She always smiles when I eat her food. And now she insists on washing my hair. She is always holding my head, and playing with my hair. She likes to hug me a lot in private, I can't count the number of times she has hugged me in one day. She is always telling me how much she loves her "little brother" and I always tell her I love her back, but I have not never called her my sister. I tell her, she doesn't need to do all of this, but she insists on doing it. I don't really know how to react to all of this, quite honestly I don't know that much about Japanese culture. I asked her if I am still her boyfriend, and she said "of course little brother". I think she means well, she is really sweet and caring. I just need help understanding how to interpret and act back to her. Can anyone help me out here? Thanks.

She calls me her "little brother"
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