Alison Davies's body was recovered from the River Humber in April two weeks after that of her son Ryan. CCTV cameras caught them falling from the bridge eight seconds apart.
Lindsay Cook and Julie Armand spoke lovingly of their sister yesterday, say
ing they did not want her to be remembered just as the "woman on the bridge". They also said they would try to raise public awareness about Fragile X syndrome – the condition that Ryan had.
"She was an independent, proud and private person. Ryan was the focus and purpose of her life," said Mrs Cook, of Stockport.
"She had a deep, abiding and consuming love for him. Without him I don't think she would have lived as long as she did. She had the double whammy of depression and Ryan's disability to cope with. And she was a single parent. She fought battles and she got tired."
At 12 Ryan had a mental age of seven and life was getting increasingly difficult as he was picked on by bigger boys, who would tease him about the way he spoke.
The sisters believe many cases of Fragile X syndrome, which is passed from mother to child and affects about one in 1,000 to 2,000 males, go undiagnosed. Ms Cook said: "It is our duty as a family to highlight this illness. Alison has given us this opportunity. We would rather she hadn't but she has."
They said that they did not blame social services, although more could have been done. Respite care had been arranged with another family for Ryan but that had not worked out and Ms Davies did not always take the anti-depressants prescribed.
"The help she got from social services was patchy," said Ms Armand. "There was no one to bounce her worries off. She was vulnerable and needed proper counselling. I am surprised she was not seen as higher risk."
Of the family only Mrs Cook's husband Andy, a father figure to Ryan, has seen all the CCTV footage of the pair on the bridge. In their final moments they looked "like two people playing", he said. "There's a sense of peace that she had made a momentous decision."
An inquiry into the deaths will take place later this year.
