TWO big-hearted big sisters from Dunfermline will be walking 100km to help their wee brother – before he's even born.

Twins Kaitlyn and Chloe Whyte, 17, were “shocked and scared” when they discovered that little Lucas, who's due in November, has hypoplastic left heart syndrome, a rare congenital defect where the left ventricle of the heart is severely under-developed.

The news came as a “devastating” blow to their mum, Claire, 34, who also has sons Cain, 10, and Harrison, 13 months.

Keen to do their bit to help, the twins, of Tay Terrace, will now walk the Fife Coastal Path from North Queensferry to St Andrews over three days from August 5 for charity Little Hearts Matter.

They'll be undertaking the challenge with Claire's partner, Lee McBride, 31, Lucas' dad.

Woodmill High pupil Kaitlyn told the Press: “We're very excited about having another little brother and were really shocked when we found out about his condition.

“We didn't even know what it was and we looked it up. We were really upset and we were worried about how it would affect him.”

Chloe, who is studying beauty care and makeup at Fife College, added: “We're doing the walk to help Lucas and other children with the same condition.

“Lee came up with the idea a couple of weeks ago and we thought it was a good idea – Little Hearts Matter have been supporting our family since we found out.”

Lucas will be born at Guy's and St Thomas' in London and will then be transferred to the Evelina London Children's Hospital. He could face major open-heart surgery, called the Norwood procedure, at just days old.

This will be followed by a second operation, called the Glenn procedure, three to four months later. A third procedure, the Fontan, could be carried out between 18 months and three years.

Claire said: “There's no cure for it and he will have half a heart for the rest of his life.

“I was absolutely devastated. It's not something you expect to hear. I had had four healthy babies with no problems and to have such big issues with this one is still devastating.

“It's probably the worst heart condition a baby can have. It's quite rare – my consultant had only come across one other baby with it.

“There's a 65 per cent chance of surviving until the age of five – if he doesn't have the surgery when he's born, he would die within days.”

Claire said she was “touched and proud” of Lee and her daughters, adding: “The walk is giving them something to focus on, otherwise we would all be sitting around worried and depressed.

“The support from Little Hearts Matter has been absolutely amazing and that's why the girls are doing it.”

To sponsor, log on to www.justgiving.com/fundraising/Claire-Whyte.