The exceptional case, filed on Monday, is being heard by a civilian court in Geneva, said Jerome Sobel, who heads Exit in the French-speaking western region of Switzerland.
The 82-year-old Swiss man, whose name has not been disclosed, had initially set the date of October 18th for putting an end to his life, Sobel said.
He informed his five brothers and sisters, two of whom then filed suit.
His plan for suicide, which requires court approval, has been put on hold while the complaint is heard.
The brothers contend that their sibling, a member of Exit for more than two decades, is suffering from temporary depression and is not acting of his own free will.
Exit says he has “multiple aged-related disabilities” and says he has written a letter attesting to “intolerable” mental and physical suffering.
Under Switzerland's right-to-die laws, patients can self-administer a lethal dose prescribed by a doctor acting for a not-for-profit assisted suicide association.
In 2014 Exit eased its criteria to allow people suffering from “multiple age-related debilitating conditions” to access their services, rather than just those with a terminal illness, reported Le Matin.
The brothers' lawyer, Francois Membrez, said this was not in line with recommendations from the Swiss Academy of Medical Sciences (SAMW), which state that the person's life should be nearing its end for them to be able to resort to assisted suicide.
“The desire to commit suicide should not be the symptom of depression,” added the lawyer.
Acting for Exit, lawyer Yves Grandjean said SAMW's recommendations were not prescribed by law, reported Le Matin.
A person's physical deterioration after the age of 80, and the worries that can cause, should mean they have the right to choose to end their life, he argued.
“There is a clear consent by this person who has the right to die with dignity,” he said.
The court decision is expected to take a maximum of three months.
There are two organizations offering assisted suicide services in Switzerland. Exit is the world’s oldest and largest organization for living wills and doctor-assisted suicide, and only offers the service to members – it has more than 10,000 – who must be Swiss or have permanent residency in Switzerland.
A second Swiss organization, Dignitas, also provides the service to non-Swiss who live outside the country.
Recent statistics showed that assisted suicide is on the rise in Switzerland, with the majority of cases involving people who are suffering from a terminal illness such as cancer.