Mie is short of both Maria and Marie. That is what both my memory, and my baby name book's says. I don't know if it's old or new, at least in Norway, but it's an old short form where I am.
And I'm from Denmark, so that's just around the corner. I also have a stepmother hailing from Norway. More specific Lofoten, but she's been in Denmark for more than 30 years, due to her first decesed husband, and she never went back because of my two stepsisters.