After watching their elder brother Simon complete eight marathons in eight days last year, Deborah and Marc Levy were inspired to keep the 2012 London Marathon in the family.
The trio, who grew up in Norwich, raised more than £2,000 for Team PB, Prostate Cancer Charity and Breast Cancer Care. The siblings' mother Beverley had preventive surgery against bowel cancer at the end of 2010. "It was quite a serious scare," Ms Levy said. "If not operated on, it would have turned into bowel cancer.
"The thought of cancer ever since has had a massive impact on our lives," added Ms Levy, who celebrated her 24th birthday the day after the race. "We also know of people who have suffered both horrific illnesses."
Distance specialist Simon was the fastest of the three, finishing in under three hours 10 minutes. His brother and sister came in inside five hours.
"The run was much tougher than I could ever expect," said Ms Levy, who had previously completed three half-marathons. "My legs wouldn't take me past a certain speed and I really wanted to give up at points.
"I had no option but for my mind to pull me through. I thought about how much we'd raised, my brothers running and the people expecting me to finish. I couldn't give up."
She added: "I'm not sure I'll be doing it ever again but for now I'm happy to say I've been a part of history."