For the past three years, Madam Tan Powi Kim, 62, cared tirelessly for her nine-year-old grandson, who suffers from a rare autoimmune condition .

Looking after him was not an easy task, as he would bite, scratch and hit others involuntarily, said the boy's father - Madam Tan's son, - Mr Goh Cherk Wui, 35.

"But no matter what happened, she never complained," he said.

On Saturday morning, Madam Tan's family - she also has two daughters - lost their pillar of strength when a black BMW car crashed into her and her husband in Collyer Quay.

She was taken to hospital where she died soon after.

It is understood that a 45-year-old man was arrested for causing death by a negligent act. He was not found to have a blood alcohol level above the legal driving limit.

The couple were loading cardboard boxes onto their lorry parked at a loading bay near the OUE Link Bridge when the accident happened just before 7am.

She had stood at the rear of the lorry to hand the boxes to her husband, Mr Cheng Kiat Yan, 68, who had climbed into the back.

It was a weekend routine the couple shared for the past 10 years, as Mr Cheng - her second husband - works as a rag-and-bone man.

The impact threw Mr Cheng off the lorry onto the ground and left him with a fractured knee.

He went home with his left leg in a cast, but yesterday was warded at Changi General Hospital after suffering from high fever, swollen arms and severe pain in his body through the night.

Speaking at the wake yesterday morning, Mr Goh broke down as he recalled his stepfather's phone call on Saturday morning.

"I remember clearly that I had just woken up. My phone rang and my stepfather was crying when he said, 'Mummy got hit, die already.'

"I knelt down and burst into tears. I couldn't accept it," said Mr Goh, who works in the shipping industry.

He went to the scene and learnt that she was already on the way to the hospital.

"I was very happy then. I thought that it meant there was a chance (of surviving)," he said.

Now, he wants to know how the accident happened.

"She was a victim of someone's actions. It is very unfair for my family," he said. "Without an answer, I just can't carry on."

Madam Tan spent most of her time by her grandson's side, as he suffers from autoimmune encephalitis, a disease in which the immune system attacks the brain. The family rarely went out of their Tampines home, much less on holidays overseas, because of his condition.

When they considered getting a maid to help, Madam Tan refused and insisted she could cope.

Mr Goh said she had also single-handedly raised three children after her first husband died 23 years ago.

His two older sisters were planning to take their mother on a rare holiday to Bangkok in June and had even bought the plane tickets.

Mr Goh said: "She sacrificed so much for us and I never got the chance to repay her. Now, she has left without saying goodbye."