While tests showed that the 18-year-old's lump was in fact just a harmless cyst, it led to both her parents checking themselves straight away for lumps. Her mum Paula found a lump in her own breast that turned out to be breast cancer, followed by her dad Ken who found a lump in his neck which was diagnosed as a form of skin cancer.
Luckily the pair were treated in time with chemotherapy treatment and radiotherapy, and have been given the all clear.
"We really have Sian to thank that we are still here today," admits Paula, 47, a deputy head teacher.
"I only checked my own breasts because we'd had such a scare after she discovered the lump. And then Ken was extra vigilant too when he saw me being diagnosed and going through treatment, that when he found a lump in his neck he went to get it looked at straight away.
"If we hadn't acted when we did, then it may have been such a different story. And we only did it because of what had happened to Sian."
Sian discovered the lump in her breast shortly before March 2009. The family were worried but tests showed that it was just a fatty lump.
"I was so relieved when the doctors told us what it was," says Sian, who has an older sister, Cara, 21.
I didn't think it could be cancer because I was so young, but I thought I should tell mum about it anyway. I know that lumps can be a sign of breast cancer, so when I told her she wanted me to get it checked straight away. Luckily tests showed that it was just a fatty lump.
But taking her daughter for tests made Paula herself think that she ought to check herself too.
"Even though Sian's lump had turned out to be harmless, it did make me think that I ought to check myself," says Paula, who lives in Hale Barns, Cheshire. "There is no history of breast cancer in our families, so I never thought that it would be a problem, but I thought I had better make sure."
But then when Paula checked her own breasts, she discovered a lump on her left side:
I never in a million years thought it would be breast cancer but I went straight to the doctors to get it checked out.
"He referred me for an appointment at the hospital but the appointment wasn't for another three months. Luckily I had a friend who worked at a local cancer hospital who suggested that I try and get an earlier one, which I did."
Paula had a mammogram and ultrasound examination in May that year and doctors broke the devastating news to her that it was breast cancer:
"I was just devastated when I got the news. It was so unexpected and came as a complete shock to us all."
Two weeks later Paula was told that the cancer had spread to her lymph notes and she had an operation to remove more breast tissue.
"It was very upsetting for both the girls and for Ken too, to see me in hospital after the operation," says Paula. "It had come as such a shock to everyone in the family when I was diagnosed, but they were all a great support for me, even though they were all so upset."
Paula had seven months of aggressive chemotherapy treatment at Christie's Hospital in Manchester, which left her sick and exhausted and she lost her hair. Then she underwent a month of radiotherapy treatment to try and stop the spread of the disease.
She finished her treatment in March 2010, and the family thought they could put it all behind them, but then just eight weeks later, Ken discovered a lump in his neck.
"I kept it quiet from Paula for a couple of weeks as she had only just finished her radiotherapy and I didn't want to worry her," explains Ken.
Had I not seen Paula go through her treatment, I may not have been that bothered about finding the lump, and might just have waited for it to go away by itself. But I'd seen that Paula's lump had turned out to be cancer, so I didn't want to take any chances with mine.
"I felt sure that it couldn't possibly be cancer, but I wanted to make absolutely sure. I went to the doctors for blood tests without telling anyone, and then when Paula had got back from her summer holiday with the girls, I told her that I'd been for tests and asked her to go with me for the results."
"I was shocked when Ken told me that he'd found a lump in his neck and he'd been having tests," says Paula. "But I thought it couldn't possibly happen again."
But it did. The doctors broke the news to the couple that he had cancer.
"We just couldn't believe it," says Paula. "It was a form of skin cancer in the glands in his neck."
Ken had an operation to remove the glands, followed by a course of radiotherapy. It made him so sick that he lost three stone in weight, and the treatment finished on Christmas Eve.
The couple are both in remission and there is no sign of the cancer coming back - but they say they owe their lives to their daughter.
"We have all helped each other out," says Paula. "Without Sian I wouldn't have checked myself, and neither would Ken, and it could have been months before we realised. By then it could have spread and it would have been too late.
Our daughter saved our lives, and we are just so thankful for that. It has been tough for her because she has had to go through her GCSE exams whilst watching us both fight cancer. But both our daughters have been such a support.
"I'm just glad I didn't keep quiet about it," says Sian. "As it meant that mum found her lump and they are both still here."
More on Parentdish:
Daughter's love letters save mum from breast cancer
A glass half full: A breast cancer blog