Katie and Chelsea Young, 11, had contracted the disease within weeks of each other even though the chances of both getting it were nine million to one.
Their family feared that one or both of them could die.
However, upon their recovery, there was no one happier than their mother Sheena, 42, who for the first time felt "fantastic" as she waved them off to school.
"When they were diagnosed it was always in the back of my mind that one or maybe both would not pull through," the Telegraph quoted her as saying.
"They are now happy and healthy girls with their whole life ahead of them," she stated.
The first time the disease was caught was in May 2001, when a nurse noticed Katie's pale and listless look. She was later diagnosed with acute lymphoblastic leukaemia (ALL).
With ALL, immature white blood cells fill up the bone marrow and stop it from making blood cells properly, leading to an increased risk of infection.
Their mother was terrified that Chelsea would also get the disease as she was an identical twin, but doctors reassured her that the chances were nine-million-to-one.
Not satisfied with what the doctor had said, the family took Chelsea for tests and were shocked when they discovered that she also had the illness.
The twins, from Brecon, Powys, were made to undergo two years of intensive chemotherapy, and they lost all their hair and wore blonde wigs.
"There were times when they were both in hospital, then one would improve, while the other worsened," Mrs Young said.
"It was tough for them and for us as a family but slowly they beat this awful disease.
"I think what got the girls through is that they had each other and they drew strength from that," she added.
Now that they have been given the all clear, the girls have grown back their natural brown locks and on September 3 started school at Brecon High.
"We've got lots of friends starting with us - everyone knows us," Katie said.
"We are feeling really happy and okay now," she added.