West Yorkshire Police said it was "extremely concerned" for the group's safety after receiving information that the woman may have crossed the border.
Khadija, Sugra and Zohra Dawood - all in their early 30s - and their nine children aged three to 15 from the city of Bradford went missing over a week ago.
Their brother is understood to be fighting with IS extremists in Syria. The group went missing after making a religious pilgrimage to Medina, Saudi Arabia, and have triggered a worldwide hunt.
Assistant chief constable Russ Foster said: "We have received information that contact has been made with the family in the UK which suggests one of the missing adults may be in Syria".
More From This Section
"Contact has been made by one of the missing women and there is an indication that they may have already crossed the border into Syria but this is uncorroborated," Foster said.
The husbands of the three sisters, believed to be of Pakistani-origin, made an emotional appeal yesterday asking them to return to "normal life", saying they "miss and love" them.
"Please contact me and please, please call me," he said.
"It's been eight, nine days you have been out and we don't know where you are. We miss you. I love you. All of you, I love you a lot. I can't live without you," Shoaib added.
"Please come back and contact me. I'm not angry, I'm not angry, I'm not fine, please come back, everything is normal. Come back to normal life please. They're young kids," Shoaib said.
British authorities estimate more than 700 Britons have travelled to Syria, with many thought to have joined IS.