The bodies of 60-year-old Arouba Barakat and 22-year-old Hala Barakat were discovered late Thursday, the state-run Anadolu Agency reported, when friends anxious that the journalist had not showed up for work alerted police.
Halla was working as an editor for the pro-opposition website Orient News.
In February 2015, Deah Barakat, a cousin based in the USA, was shot dead alongside with his wife and her sister in North Carolina.
In a Facebook post, Orouba Barakat's sister Shaza said they had been "assassinated" by the "hand of injustice and tyranny" - a reference to Bashar al-Assad's regime.
Other sources said the Assad regime is a prime suspect in the killings of the Barakat women.
The Barakats were close friends of Kayla Mueller, an American humanitarian aid worker who was kidnapped by Daesh in Syria in August 2013 and killed 18 months later.
The US State Department released a statement condemning the murders and saying officials will "closely follow the investigation", using alternative spellings for the victims' names.
It is not the first time for tragedy to strike the family: a cousin - Deah Barakat - was in Chapel Hill, North Carolina, in 2015.
Turkey has become home to nearly three million Syrian refugees since the outbreak of the Syrian civil war in 2011, many of them opponents of Bashar Al Assad's regime. "Turkey must ensure the protection of Syrian journalists who have fled to the country seeking safety".
A Syrian journalist who reported on the fight against IS was shot dead a year ago.