Molly Campbell went missing from school on Friday. It is believed she was taken by her father and elder sister and flew with them to Lahore.
Police say they want to reunite the girl with her mother, Louise Campbell, her legal guardian.
Last night reports quoted Molly's grandmother as saying she feared the schoolgirl had been taken to become a child bride.
Violet Robertson, 67, said: "It's just terrible. Molly is only a little girl. It's an arranged marriage.
"She doesn't know the man. He's 25. Molly doesn't want to go to Pakistan. She wants to stay with her mum."
Molly, also known as Misbah Iram Ahmed Rana, was last seen at 10:50am on Friday in the grounds of her school, the Nicolson Institute in Stornoway.
Northern Constabulary believe she was met there by her sister, Tahmina, 18, and the pair took a taxi to Stornoway Airport before flying to Glasgow.
They then met their father, Sajad Ahmed Rana, and all three boarded the 2:55pm Emirates flight to Lahore.
Although her father is known to live in Lahore, it is thought Molly may now be living with his relatives in Karachi.
Mr Rana is known to have visited Stornoway on Thursday but left for Glasgow the same day.
Northern Constabulary is working with Interpol, the Crown Office and other agencies, including Western Isles Council, to try to track down the girl and return her to Scotland.
The force said it was investigating a possible offence under the Child Abduction Act. Chief Inspector Murdo Fraser said:
"We will be doing all we can over the coming days to ensure Molly is reunited with her mother as soon as possible."
Molly and her mother are believed to have moved recently to Stornoway.
Ms Campbell is thought to have divorced her husband several years ago, gaining custody of Molly, while Tahmina remained with her father.
Police said Mr Rana had not attempted anything similar in the past, but could not say if Molly was taken away under duress.
However Ms Robertson was reported to have said: "Molly's mum has been running from Sajad for a long time now. He won't hand Molly back - never. I'm scared that's her gone for good."
The pensioner also claimed Molly's father had tried to snatch his daughter a few months ago, prompting her mother to move her to another school.
"She enrolled her under a fake name because Sajad would recognise her as his daughter if she called her Molly," said Ms Robertson.
Ms Campbell and her daughter fled their Glasgow home 18 months ago to hide from Molly's father, according to reports
They set up home in Drummore, near Stranraer, but Mr Rana is reported to have tracked them down with the help of a private investigator and they then fled to Lewis.
The couple were married in a Muslim ceremony in 1984 and had four children - Omar, Tahmina, Adam and Molly - but the marriage later fell apart.
Denise Carter, of Reunite, a charity specialising in cases of international abduction by parents, said: "Unfortunately, it is a growing global problem.
"Last year alone, we dealt with 300 to 400 new cases of children being taken from the UK and a significant percentage of those involved Pakistan."
Ms Campbell, 38, is today expected at a news conference to make an appeal for her daughter's safe return.