Mr McVey,
I have Geoffrey Marcus' book, and I cannot find any information within it relating to Wilde's letter to his sister. (Unless either I or the index has missed a reference!)
However, Marcus does say: "Alone among the Titanic's officers, Wilde was reluctant to accept the new appointment; for some time, in fact, he hesitated; and only when a number of his friends declared that he would be mad to refuse this chance did he finally, and with much misgiving, accept it."
Marcus indicates in his chapter notes that the above is "private information" so it is not clear how he came across this tidbit.
My book is printed May 1978, so it's possible that if a later edition was produced, Marcus may have added something to the above.
(BTW, I also checked in Michael Davie's book "The Titanic - The Full Story Of A Tragedy" without any luck. I can't find my copy of ANTR at the moment; maybe it's in there?)
Hope this is of some assistance.
Regards,
Paul.