Laura Siegel criticised the methods and morals of the DC Comics owners, saying that she will not be "bullied or deterred".
She accused Warner Bros of continuing to attack her lawyer, Marc Toberoff, in order to delay handing over the money that it has been ruled belongs to her family and that of co-creator Joe Shuster.
"I refuse to be bullied or deterred from enforcing my family's rights, and fully support my attorney who has tirelessly defended them," she wrote in the letter addressed to "Superman fans everywhere".
"Warner Bros' smear campaign has only made me more determined than ever. We have the right to the attorney of our choice, which is none of Warner's business.
"Marc Toberoff has stood by us through thick and thin and we stand by him. We love and respect Marc who has poured his heart and soul into helping us obtain justice in court, tenaciously litigating on our behalf against the legions of highly paid corporate lawyers that Warner Bros has at its disposal.
"It was Marc's dedication and skill that brought us victory in federal court, which Warner Bros. desperately hopes to derail with its vindictive and baseless lawsuit. Make no mistake, Warner Bros is attacking our attorney to attack us and for no other reason.
She pointed out that Warner has spent $35 million (£22 million) on corporate lawyers rather than investing in a settlement. Toberoff has not received any money from her family since 2004, and will not until the case is won.
"The very attorneys who are lining their pockets with millions in fees accuse my attorney of profiteering..." Siegel continued. "The notion advanced by Warner that he is trying to own or control Superman is ridiculous."
Laura Siegel has taken charge of the legal proceedings following the death of her mother Joanne Siegel at the age of 93.