A UN official told AFP news agency that the seven-day-old and two-month-old girls had died in Idlib province.
Some 2.8 million people have sought shelter in the opposition stronghold after fleeing their homes during the country's decade-long civil war.
Many live in worn-out tents without warm clothes and fuel for heating.
The UN has warned that the situation is getting worse due to a severe economic downturn in Syria that has seen food prices double in a year, as well as a shortage of funding to provide winter aid and increased needs.