Hi,
while searching for birth family of a US adoptee about a year ago I had to do with the adoption office in Stuttgart. The lady there was quite helpful within a few days so that I could connect the US American with his German half-siblings (5 altogether).
Did you know that children born to unmarried mothers were listed on the grandparents' registers? That was the case only in the state of Baden-Württemberg and will certainly apply to your family also. If so, it would be quite easy to find possible (half-)siblings.
Feel free to contact me at find-our-fymily (at) t-online.de. I'll be happy to help you. For further general information on how to get certified copies of certificates and find birth family please see my many former posts on this board.
Gaby