Faith Williams, the baby girl that survived an emergency surgery of separation from her baby sister Hope remains seriously ill at London’s Great Ormond Street Hospital.

Hope died shortly after the surgery because her lungs were too small to sustain her breathing. Faith is on a ventilator while her parents Laura, 18 and husband, Aled, 28, are at her beside. If her condition improves, surgeons hope to operate again to close the wound in her abdomen where they were separated.

“Faith Williams is a very sick little girl. Great Ormond Street Hospital is doing everything it can for her, and we continue to offer her the highest possible levels of care,” a statement released by the hospital read.

The twins were delivered by C-section on November 26 at London’s University College Hospital. They were joined at the chest and shared a liver but had separate hearts, which made doctors believe there is a good chance that the operation would succeed. Also the girls had to be separated because a blockage in their joined intestine appeared and that made the surgery more than necessary.

The surgery involved a team of more than 20 doctors, nurses and other health care professionals. It lasted for about 11 hours.

“The operation done on Hope and Faith was one of the most complex and challenging we have ever faced,” Professor Agostino Pierro, head of the surgical team, said.

Laura Williams is thought to be the youngest mother of conjoined twins in Britain. Doctors warned her and her husband that their babies might not survive after a 12-week scan revealed the problem but they refused to consider a termination.