Meghan Markle's half-sister said Meghan is using her "depression as an excuse for treating people like dishrags" on Inside Edition.
Samantha Markle said Meghan knew "what the media would be like" for her and Prince Harry before marrying into the royal family.
"We've seen how that ends," the actor added.
Thomas Markle said the letter was like "a dagger to the heart."
Families of celebrities can find themselves caught in the "reflected glory" of fame.
Samantha Markle appeared on several talk shows prior to Meghan and Prince Harry's wedding to talk about their relationship.
"I would do it for you for 1,500 my minimum is much higher," Samantha Markle said.
"Act like a humanitarian act like a woman! If our father dies I'm holding you responsible, Meg!" Samantha Markle said.