Evie and Chloe Church have a rare illness which can cause them to die if they hear a loud noise or experience sudden shock.
Every morning the sisters, aged nine and 11, have to be woken from their sleep with the greatest of care.
The girls have the rare medical condition Long QT Syndrome, which causes an abnormality of the heart's electrical system.
As a result of their illness, any sudden rush of adrenaline or a sharp rise in blood pressure could prove fatal.
Even any type of exercise - especially swimming - could be fatal.
Samantha Church, the girls' mother, said: "Sometimes it just feels like I'm living with a time bomb, terrified it today will be the day it goes off.
"It breaks my heart not knowing if they are going to be here tomorrow or in five years time."
Long QT Syndrome affects one in 7,000, and is usually genetic.
Mrs Church, 41, said: "My mother-in-law had the condition and had to have a defibrillator fitted.
"We had the girls checked from an early age but initial tests proved inconclusive."
It was doctors at Swansea's Singleton Hospital who picked up the little-known condition during an ECG scan of the youngest daughter, Evie, in July this year.
Mrs Church said: "Chloe and Evie have to be woken up really carefully in the morning and have to skip PE lessons at school.
"They exercise using the Nintendo Wii, but have to be strictly supervised.
"They're both on beta blockers for life. It's been incredibly tough for them, but they've coped remarkably well."
Luckily for the girls, they are fairly safe from harm as long as they can prepare for loud noises.
It meant they could go with their friends to a fireworks party this week and enjoy the bangs like any other child.
"I want Chloe and Evie to try and have as normal a life as possible – I can't wrap them up in cotton wool," added Mrs Church.