Considering what happened to your family, you have an enormous amount of information. Congratulations! You deserve a big pat on the back.
All of our research in the genealogy category of Just Answer is done online, and I have access to all online genealogical records. However, not every record is online. I do not think I have ever seen an adoption record online. In the United States, most of the adoption records are sealed by the court and we cannot access them. You are probably going to have to hire a local researcher to do on site research, or an attorney to locate the records, and petition the court on your behalf. I wish there was more I could do to help you.
I did locate one record that could possibly be your sister Heike, assuming her name was not changed.
U.S. Public Records Index, Volume 1
Name: Heike M McMillan
[Heike M Fischer]
Birth Date: 3 Sep 1958
Phone Number:(NNN) NNN-NNNN/p>
Address: 323 Ashley Brook Dr., Lawrenceville, GA,NNN-NN-NNNN(1996)
[3902 75th Terrnce East, Sarasota, FL, 34243 (1994)]
[1501 Moreno Ave, Fort Myers, FL,NNN-NN-NNNN
The following link will take you to a web page full of information about German research.