It's very likely that Fischer's farther was Paul Nemenyi. And Joan's father was indeed Hans-Gerhardt Fischer. Note that Joan was five years older than Bobby.
Indeed - look at the picture of Paul Nemenyi in the link above and make up your own mind...
Reminds me of a recent news story I saw which suggested Frank Sinatra "might" be the father of Mia Farrow's son Ronan - and not Woody Allen:
http://www.telegraph.co.uk/news/worldnews/northamerica/usa/10350935/Mia-Farrow-Woody-Allens-son-Ronan-possibly-Frank-Sinatras.html
Indurain : This has nothing to do with Bobby's sister, but gives info for you about address and date ! I hope I can find more.
Wow that is interesting! How did you come across this letter?
"..your little chess-playing boy.." - aww that's sweet.