I just had a look into the book Der in Bayern immatrikulierte Adel XVII published in 1988. On the pages of the Thurn und Taxis family I found the information that Princess Fernanda of Thurn und Taxis, sister of the late Fürst Johannes and aunt of the current Fürst Albert, adopted a child in October 1966 at Kings County, New York. The child's name is Marina Editha Theresia and she was born on May 12th 1966 in New York. So Princess Fernanda adopted the girl only five months after her birth. The book says that Marina is bearing the title "Prinzessin von Thurn und Taxis" and in 1988 (Marina was 22 by then) she still lived with Fernanda in Munich.
Another interesting fact is, that Fernanda's marriage to Franz Joseph of Hohenzollern had already been annuled when the adoption took place. So Fernanda was a single mother as she didn't remarry.
Does anybody have further information about Princess Marina? Paul Theroff doesn't mention her on his sites which is quite surprising for me. Any pictures?