German half-sisters, no luck in finding them

I've been looking for several years now for my two half-sisters. Sylvia, the elder of the two, was born May 2nd, 1965 in Hoxter and Silke was born September 13th, 1967 in Hannover. Their last name at birth was Wedemeyer, but that may have been changed to Schroeder. They were adopted by our maternal grandmother when my mother married my father and moved to Canada. My mother believes that their last name would have been changed at that point. Our grandmother's name is (was?) Hannelore Magdalene Schroeder. She may also go by her maiden name of Wedemeyer, Klinger or Ahrens, the surnames of her two other husbands. Any advice? Mona