John and Robert McIlwain are believed to have been suffering severely from pulmonary fibrosis, after being exposed to asbestos.
Robert, 73, had suddenly become very frail, according to neighbours, and had taken to using his disabled brother's mobility scooter.
Robert was caring for his 71-year-old brother and neighbours believed he did not want to leave his sibling living alone.
Police went to their house in Edinburgh on Thursday after receiving a phone call from one of the brothers claiming he was going to kill himself.
The bodies of the two men were found slumped on top of a firearm, with gunshot wounds to the stomach, at around 6pm in Lockerby Cottages, Gracemount.
Resident Anne Parker, 83, who lived next door to the brothers, said: "They had planned it pretty well. They had no family, and they had asked around to see if we could witness them signing their will a few weeks ago.
"My husband, Frank, was meant to do it last week, but it was not ready on time.
"There was no evil in them, they were as good as gold. They were very thoughtful."
She added: "I can only imagine that they had planned it and that it was done out of love.
"I think one would have done it so that the other wasn't alone - they had no-one.
"Bob would be the carer for Jack and he would have been scared that Jack would have been alone if he had died."
A police source denied any suggestion that one brother had shot the other, insisting that it was a double suicide.
A card placed in a bunch of flowers at end of their garden read: "RIP Bob and Jack. Two lovely gentlemen. Its been a privilege to know you and call you friends. You will be missed by us all."
The two brothers are believed to have been bachelors and had worked together as gardeners before they retired due to ill health.
John, known locally as Jack, used a wheelchair and had been housebound for the past year. He was rarely seen by neighbours.
His older brother Bob had apparently become ill in recent weeks and was receiving oxygen to help ease breathing problems related to pulmonary fibrosis.
Neighbour Irene Philp, 78, said Bob had looked much thinner in the past few weeks and had not been out as much.
She said:"The last time I saw him he was on his brother's mobility scooter. I was in someone's garden and 'Hi Bob' but he didn't answer.
"They could have had so much more help than they had."
The property where the men died is owned by a charity, Lockerby Trust, which caters for "distressed gentlefolk who have fallen on hard times".
Set up in 1894, Lockerby cottages is a quiet, leafy cul-de-sac off Lasswade Road, Edinburgh. An imposing gateway leads on to 16 semi-detached houses and two bungalows occupied by 34 people.
A Police Scotland statement said: "Police in Edinburgh attended a property in Lockerby Cottages near Gracemount at around 5pm.
"On gaining entry, two elderly men were found seriously injured and were pronounced dead at the scene.
"Inquiries are continuing but police are not looking for anyone else in connection with this incident."