Is it weird that me and a friend got so close after 3 months that we consider each other brother and sister?

JUST1NC
My friend was going out with this exchange student and when we finally met we had a great connection straight away at first it bothered him but I quickly explained that I only like her as a friend and (I told her already and she felt the same) and only a few months later when she went back to her country she wrote a letter to her friends and called me her brother and we have been calling each other brother and sister since
Is it weird that me and a friend got so close after 3 months that we consider each other brother and sister?
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