Basant Pant, a neurosurgeon in Kathmandu, said Ganga and Jamuna Shrestha had not been given the "best care" after the marathon operation that separated them in the city-state seven years ago.
"Have the twins been given the best care? The answer is no," Pant told the Straits Times newspaper.
The twins did not receive the necessary physiotherapy and occupational therapy, Pant said, adding that doctors had failed to bring the girls back to Singapore as promised to repair their skulls, the newspaper reported.
Pant's comments followed the death of Ganga last week in a Kathmandu hospital from respiratory problems.
Jamuna needs reconstructive surgery on her skull and Singapore doctors should provide the treatment, said Pant.
"Singapore started the show, so it should finish it," he said.
The twins were born with fused skulls and were separated by a team of neurosurgeons at Singapore General Hospital in a 97-hour operation performed free of charge in 2001.
The girls returned to their parents in Kathmandu, but the operation left Ganga with brain damage and Jamuna unable to walk. They returned to Singapore in 2005 for additional treatment before returning to Nepal.
Keith Goh, who led the team that separated the girls, told the newspaper the they were not fit last time he saw them in 2005 for reconstructive skull surgery.