Astonishingly, the cancer had not been diagnosed until just a few weeks before her death in 1993. That is probably when she guessed that Sophie would kill her. After all, she had killed her own mother almost half a century before.
It would be fair to say that mercy killings run in the Jones family. Two generations of them have killed their mothers and a third will be told, at the right time, that it is more humane to end a person's suffering than it is to watch them die in agony.
Sophie Jones, 40, was one of the dozens of people to come out of the woodwork in March when Derek Rowbottom, 44, from Greater Manchester, admitted giving his 80-year-old mother, Alice, an overdose of morphine to end her suffering from cancer.
He told the Independent at the time that he had opened a "Pandora's box" of guilt, released a flood of admissions from people who had secretly killed their own relatives. Some had given overdoses; others had withheld medication. Two told him they would come forward with public admissions if Mr Rowbottom were charged. His case is still being considered by the Crown Prosecution Service.
But none came forward with a story more moving than Sophie Jones's. "I feel complete sympathy for Mr Rowbottom," she said, "because my mother killed my grandmother and I, in turn, killed her."
Sophie's grandmother, Anne, was a nurse working in the Midlands when, before the outbreak of the Second World War, she developed liver cancer.
"It was before I was born, but when I was growing up, my mother told me about it," said Sophie, a mother of two from north London. "She had shrunk to no more than a yellow skeleton. My mother said her skin was stretched thinly over her bones and she was crying out in agony.
"She recalled one occasion when she was in so much pain that she was literally trying to climb up the walls. I know that climbing the walls is an expression, but my grandmother was actually doing it.
"My mother was a nurse and she was caring for my grandmother at home, so she and a doctor got their heads together and decided to put her out of her pain. When I was a girl, she always told me she couldn't bear to stand by while her mother was suffering so badly.
"She likened it to the Good Samaritan - it was easier to walk on by than to help someone in distress but that didn't make it right. She decided to be a Good Samaritan. I don't know how she administered it, but she gave her mother an overdose of morphine which, nearly 50 years later, is what I gave to her."
No one knows how many people are killed out of kindness. Very few confess, so the collation of statistics is impossible.
"They will just be there under the numbers convicted of murder or manslaughter," said a Home Office spokesperson "or, perhaps, aiding and abetting someone's suicide, which is an offence under the 1961 Suicide Act. That carries up to 14 years' imprisonment."
It would seem, however, that mercy killing is a lot more common than was previously thought. After he admitted killing his mother, Mr Rowbottom was astounded by the number of calls of support he received.
"I seem to have opened something of a Pandora's box," he said. "I had dozens of calls from people who said they did the same thing because they simply could not bear to see their loved ones in so much pain.
"There needs to be a change in the law that allows people who are terminally ill and in terrible pain to die with dignity. It seems as though a lot of people have done this but haven't had the courage to admit it because of the law as it stands. Now some of them are coming forward and it will be interesting to see what happens to us."
Sophie Jones is not coming forward. She has two young children - a girl and a boy - and if she made her deed public, she could be charged with murder.
"I have no regrets about what I did," she said. "It was in 1993 when they finally told my mother she had cancer. Her husband had died and she had re-married and was beginning to live her life again to the full. Once they told her she was riddled with tumours, she sank very quickly.
"She had had stomach problems for years and had been reduced to eating Complan and baby food, but they had failed to diagnose cancer. At the end, she couldn't even swallow the distalgesic painkillers they were giving her. I demanded they gave her morphine because she was in so much pain.
"I was caring for her at her home and I was supposed to administer the morphine by spoon but it hurt her too much to sit up, so I gave it to her in one of those big baby syringes orally, without a needle.
"By this time, she was in terrible pain. She would slip in and out of consciousness and would occasionally look at me with fear and panic in her eyes. In the last two days she came round and was talking about getting better. I said we had to bear in mind that she might not get better. Her eyes were bright and she looked at me and nodded, then she went to sleep. "I got the impression then that she was relieved, that she was exhausted but she had done her fighting and could let go now."
Perhaps Beatrice Jones's relief was based on a guess that Sophie would not let her down. She had always made it clear that her daughter was to do what she had done if the occasion should arise.
"She had been screaming like a rabbit," said Sophie. "I had never heard my mother scream. That's when I made up my mind to do it.
"I put 10 times the dosage of morphine into the baby syringe and I gave it to her. She stopped thrashing around and she was resting in peace. She lost consciousness and died the next day.
"I had a tremendous sense of relief. I know I did the right thing. Imagine if it was someone you loved - could you just stand there and let them suffer like that? Do you think they would thank you for it?"
Sophie certainly would not. She admits to harbouring fears about her own end, given that her mother and grandmother had developed cancer.
"My daughter is too young to understand yet," she said, "but as soon as she is old enough, I will gently explain what happened with her grandmother and great-grandmother. And I will tell her that if she ever finds herself in the same position with me, she too should do the right thing and, if I get the chance, I will thank her for it."
The names of the women in this story have been changed.