Yes, I have also posted this on the USA, Austria and Switzerland Forums. To be honest, it's possible that both Susanne's brother and sister are no longer with us (I don't have their dates of birth, but there again you never know!...) and I have no information regarding possible children. As it's only surviving relatives I'm interested in, there's probably no point in going right back to their father.
However, I have now found some additional information on the mother of all three which may be of some use in the future:
She was FRANZISKA SERAPHINE ERNESTINE MARIA KLEIN, and she died a widow on 5 March 1962 at 15 Pelikangasse, Vienna, Austria. However, she also had an address at 10 Downs Court, Wimbledon, London, SW!9.
Between 1948-1958 she had been living at Courtfield Gardens, Kensington, London.
When her Will was administered, all her effects were left only to Susanne (although I do know that both Sue's brother and sister were alive in 1986).