Prince Harry's tales of fights with Prince William and his infamous Nazi costume don't do much for Thomas Markle ... because Meghan's dad isn't going to read his son-in-law's new book.
Meghan's half-sister, Samantha Markle, tells TMZ ... her dad -- and other family members for that matter -- think the book is fiction and a money grab. Samantha says her dad is actually more upset with Meghan than Harry. She says the bad blood goes way back to their wedding, where Thomas feels he was iced out by his daughter ... and Harry just went along.
Samantha says her dad believes many of Harry's allegations in the book are rooted in jealousy over his brother's status.