Conjoined twin Faith Williams passed away to be with her sister, Hope, on Christmas Day, three weeks after a separation surgery.
The tiny girls had been born to Laura Williams, 18, and husband Aled, 28, of Shrewsbury, Shropshire, on November 26.
The twins, had been joined from the breastbone to the top of the navel, and shared a circulatory system and liver, reports the Sun.
A team of 20 doctors, at London's Great Ormond Street Hospital, had crossed their fingers hoping to successfully separate the girls, who had had their own hearts and lungs.
Following a blockage in the their intestine, the experts had taken 11 hours to complete the operation on December 2 and 3.
While Hope, the younger twin, had died soon after the medical procedure, owing to her small lungs that were unable to support her breathing, Faith had been predicted to have a 50 percent chance of survival.
Leading surgeon Professor Agostino Pierro explained that Faith's tragic demise had not been "not unexpected".
He said: "We were always clear that Faith was very sick. She required the full range of skills of our intensive care staff, and underwent a number of further procedures.
"However, she succumbed to the complexities of her condition. While this is a sad outcome, it is not an unexpected one."