FitzGerald told the court it discusses the alleged abuse.
He said the letter was "new material that the defence has become aware of since previous trials."
FitzGerald is seeking to cross-examine the stepson about the letter, despite the legal system's preference that alleged victims of sexual crimes should only be cross-examined in court once to reduce the emotional and psychological impact on them of recounting their alleged experiences.
FitzGerald argued that should hold less weight because "we have had an indication that the [alleged victim] was willing to give evidence."
The prosecution was sent the letter on Monday and is considering it.
In previous trials, the court heard the father of the alleged victim – the woman's husband – came home and discovered the two of them kissing on the couch covered by a blanket.
The court heard he sided with the woman and kicked the son out of the house, which caused the alleged victim years of anguish.
The woman has maintained her innocence throughout the lengthy legal proceedings and her two stints in jail between conviction and appeal, with the first trial wrapping up in 2016.
She is charged with two counts of gross indecency with a person under 16 years and two counts of sexual penetration with a child over the age of 10 she knew to be her stepchild, taking place in the mid-1980s.
Jury trials are not taking place in Victoria indefinitely due to Covid-19 restrictions.
The trial continues.