Sisters Charlie-Marie and Emma-Jean Skinner have baffled doctors with their survival after they were both diagnosed with the rare tumours.
Four-year-old Emma-Jean has two tumours on her heart, and Charlie-Marie has a tumour so big that doctors say they have never seen one that size before.
Amazingly she is still alive because one of her heart chambers has grown bigger, to compensate for the size of the tumour.
Their mother Heather, 27, who is married to Andy and studying to be a social worker, says: "We were devastated when the girls were diagnosed.
"Charlie-Marie was diagnosed before she was born after doctors spotted the tumour on the scan. They told us she wasn't going to survive, so the fact she is still here is amazing.
"Then a few months after she was diagnosed, Emma-Jean was tested too, and they found two tumours on her hearts.
But they don't let it get them down. The doctors are amazed at them. They can run around and play just like their friends.
The devastating news was given to Heather when she was 28 weeks pregnant. She went for a scan at hospital and doctors spotted a mass in Charlie-Marie's heart.
She was referred to St Mary's Hospital in Manchester and doctors told her that it was a tumour that was going to kill her unborn baby.
"We were absolutely devastated. They told me that I needed to terminate the baby as there was no way she would survive," explains Heather.
"The tumour was so big it was going to kill her. It was covering the whole left side of her heart and they said they had never seen one so big.
'But we went home and I knew there was no way I could kill my baby. So we decided not to go ahead with the termination and instead we waited to see what would happen.
Every time she stopped moving I thought that was it, that we had lost her. We even bought her a blanket and a little outfit to bury her in. The midwife told us to get as many memories as we could, so we bought casting kits to take casts of her hands and feet when she was born to remember her by.
Charlie-Marie was born by Caesarean at Blackburn Royal Infirmary in January last year, weighing 6Ib, 5oz.
The couple, who have four other children, twins James and Jessica, nine, Jennifer, eight, and Alfie, two, had her christened and she was taken down to special care.
"The doctors didn't think she had long left, so they wheeled me down to see her. But as the hours went by, she hung on. They couldn't believe it," says Heather.
Amazingly Charlie-Marie was still alive after four days, so the couple were allowed to take her home. When she was five months old she had tests which showed she was suffering from Tuberous Sclerosis, a disorder which causes tumours to grow in the brain, nervous system, kidneys and heart.
"We were devastated when Charlie-Marie was diagnosed. We were all tested as it is a hereditary condition," says Heather.
"Andy has the condition too, although he has only ever had two lumps removed from his knee and arm and he wasn't diagnosed at the time.
"All the other children were fine, apart from Emma-Jean who has also been diagnosed with it.
"Scans have shown that she has two tumours in her heart and one in her brain too."
The couple are now taking one day at a time, as there is a possibility that if the tumours grow more they could put fatal pressure on organs, or they can turn cancerous.
Heather, who is raising awareness for the Tuberous Sclerosis Association, says:
They are both developing normally at the moment and the doctors can't believe how well Charlie-Marie is doing as her tumour is so big. They expected her to have fits due to the size of it, but so far she is doing fine.
"They are just normal little girls at the moment, which we are so grateful for. They have check-ups every six months and we don't know what the future will bring for them both.
"But at the moment no-one would know they both have tumours on their hearts. They are so full of life, they are our two little miracles."
What a brave family.
Good luck Charlie-Marie and Emma-Jean.