Winnie Madikizela-Mandela, who is claiming ownership of the former president's rural home, was trying to stop the Mandela family from holding talks at the Qunu property without her permission.
The meeting is planned for Friday.
In a report published in a local weekend paper Madikizela-Mandela had warned that should the meeting go ahead, "violence is likely to erupt and thereby tarnish the good name of both myself and the late Mr Nelson Mandela".
Last year Madikizela-Mandela launched a legal challenge against Mandela's will.
Also Read
The Nobel Peace laureate left the house in southeastern South Africa to his family trust "for the benefit of the Mandela family and my third wife (Graca Machel) and her two children".
But Madikizela-Mandela claims the house -- built on a large estate where the revered South African leader is buried -- was acquired by her in 1989 when Mandela was still in prison.
Judge Zamani Ntlangulela dismissed Madikizela-Mandela's application regarding the meeting on the grounds that there is no full owner of the Qunu property and that Madikizela-Mandela had failed to include the executors of Mandela's will in her application.
Mandla told reporters after the ruling that the meeting will continue as planned.
"We want to emulate the footsteps of our grandfather who believed" in solving disputes "in a peacefully manner."
The revered anti-apartheid hero stated in his will that he wanted the Qunu homestead to "be used by my family in perpetuity in order to preserve the unity of the Mandela family".
The couple divorced in 1996 and she was not named in his USD 4.4 million (3.5 million euro) will which was released last February after his death in December 2013.