Conjoined twin Faith Williams died on Christmas Day, 23 days after the death of her sister Hope.
The sisters were joined at the chest when they were born by Caesarean section 26 November. Hope died in an operation to separate them shortly afterwards.
Faith's mother Laura, 18 and father Aled were at Faith's bedside when she died.
Great Ormond Street Hospital said Faith's death was not completely unexpected but was nonetheless very sad. Hope died because her lungs were too small to support her breathing.
A hospital spokesman 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."
Mrs Williams is believed to be the youngest mother of conjoined twins in Britain.
When Faith and Hope were born they weighed 10.5lb and were joined from the breastbone to the top of the navel and had a shared liver but separate hearts.
December 2008 | 
Conjoined Twin Faith Dies
