They accused Frank Gilford, who has consistently called for the death penalty if the women are found guilty, of "usurping the role of the public prosecutor" by asserting the nurses' guilt without the evidence to do so.
They made clear their intention to ask the Saudi courts to verify if Mr Gilford is an heir in the will of his dead sister, Yvonne, 55. If he is not, he would have no right under Islamic law to demand the death penalty, according to the Salah Al-Hejailan law firm in the Saudi capital, Riyadh.
Earlier this week, it was reported that Ms Gilford's will left her estate to three children whose parents had become her friends, with another friend and a godson among the beneficiaries. Media reports said the will excluded her brother Frank, who has shown no sign of softening his view that the women, if convicted, should face the death penalty.
Ms Gilford was found dead in December at the King Fahd medical complex in eastern Saudi Arabia where she worked with the two women accused of her murder, Lucille McLauchlan, 31, from Dundee, and Deborah Parry, 38, from Alton, in Hampshire.
The Saudi law firm said last night: "Frank Gilford has no business insisting repeatedly and emphatically, in the face of the contrary evidence, that the two nurses are guilty of murdering Yvonne Gilford."
The statement claimed that with the British women's confessions to police "discredited" following allegations that they were threatened with rape if they did not confess, the prosecution has made no assertion of other substantial evidence against them.
"Only Mr Gilford continues to say so, and with no sufficient basis, thereby usurping the role of the public prosecutor," the lawyers said.
Even the judges, the lawyers claimed, had suggested he was failing to take due account of the circumstances in which the confessions were obtained.
"It does not help the cause of justice for Mr Gilford's own statements and statements made in his name to be aggressive and inflammatory, and for him to misconstrue every single action or statement by or on behalf of the nurses as an admission of guilt."
The women's lawyers urged him to "step back" and allow the authorities to find his sister's killer or killers, and said his behaviour was "disruptive of public order" and prolonging the nurses' detention.
Declaring their legal move over the dead woman's will, they said evidence had come to light that he was not an heir: "How ironic that Mr Gilford may be keeping himself before the spotlight condemning the nurses, when he seems to have no right under Islamic law to assume this role," the law firm said.Reuse content