Emma Garwood, 38, and husband Mark, 35, were left devastated after they were told that their four-year-old daughter Megan had the cancer.
But just a week later doctors found out that Megan's twin sister Gracie also had acute lymphoblastic leukaemia (ALL).
Both the girls are still battling the disease after five months treatment at Addenbrooke's Hospital, Cambridge.
Mrs Garwood, who also has a two-year-old daughter Martha, said: 'To see both girls in a twin room at Addenbrooke's undergoing the same treatment side by side was an absolute living nightmare.
'I struggle now just to talk about it without getting upset. To be told that two of your three children have cancer is unthinkable and you begin to wonder what you have done to deserve it.'
The couple's ordeal began last August when Megan began looking tired and off colour at home in Colchester, Essex.
Her parents took her to their doctor who immediately referred her to Colchester General Hospital for further tests.
She was then transferred to Addenbrooke's Hospital after the leukaemia was diagnosed.
Mrs Garwood said: 'We had just come back from holiday and suddenly Megan couldn't walk or do all the usual lively things she normally did.
'It all seems like a dream sequence. I still just assumed she had a bug. You never think anything this bad could happen to you.
'But suddenly a doctor says your daughter has cancer and moments later we are being rushed to Addenbrooke's in an ambulance with all the blue lights flashing. And then at midnight you watch your daughter begin an intense course of chemotherapy.
'The strange thing was that Megan was the only one who wasn't panicking. She seemed to be doing fine - maybe it was the adrenalin.'
As the couple came to terms with the diagnosis, doctors reassured them that the chances of Megan's twin sister having the same cancer were slim.
But just a week later Mrs Garwood's sister called to say Gracie, who was staying with Mrs Garwood's parent, seemed tired and unwell.
Mrs Garwood said: 'We took Gracie to Addenbrooke's and asked if she could be checked.
'She thought she was only there to visit her sister, but they took one look at her and before we knew it she was moved into the bed next to Megan for treatment.
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