Ashleigh, six, and Alisha, three, were both born apparently healthy.

But their mother Jayne Lennon, 35, went to doctors to get Ashleigh checked out after she started suffering epileptic fits aged three-and-a-half.

She trawled the internet to find out why her daughter had suffered the fits and found there was a possible link to Batten’s disease, an inherited neurological condition. However, she dismissed the thought because the disease is so rare.

However, tests discovered Ashleigh did have the progressive condition, which slowly shuts down brain functions.

Ashleigh has been unable to speak or feed herself since she was five. She has to be tube-fed and uses a wheelchair. She is also going blind.

When Ashleigh was diagnosed doctors also told Mrs Lennon her Alisha had a 25 per cent chance of having Batten’s disease, but they did not test her for six months because they were worried about the impact it would have on the family. Subsequent tests showed she too had the condition.

Mrs Lennon, from Lancaster, who has another daughter, Lucy, 14, told the Daily Mail: “It is heartbreaking watching Ashleigh deteriorate and knowing that Alisha will have to go through that too.

“I know that soon I will have one daughter instead of three, and we are just making the most of each day that they are still here. It has been especially hard for Lucy to deal with as she has seen both her sisters born perfectly healthy.”

Alisha’s condition is not as bad because the disease has not yet developed so fully.

Lucy was not tested as she has a different father.

The sisters have what is known as late infantile Batten’s disease, in which symptoms develop between two and four years old. Those who develop it are unlikely to survive beyond their teenage years.

Mrs Lennon thanked the Rainbow Trust Children's Charity for its support. It helps families living with a life-threatening or terminal illness.

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