The Dogan news agency identified the detained man as Ahmet Barakat, adding that he was a distant relative of the two victims.
Police had detected him using surveillance footage from the Uskudar neighbourhood where the mother and daughter were killed, Dogan said.
Orouba Barakat, a former member of the opposition Syrian National Council, was known for her backing of the uprising against Syrian President Bashar al-Assad. But she was also critical of opposition groups.
Halla Barakat was a journalist with Orient News, a Syrian opposition broadcaster, Turkish media outlets had reported earlier.
This is not the first time opposition figures and journalists from neighbouring war-torn Syria have been targeted in Turkey.
Naji Jerf, a Syrian filmmaker who worked on exposing life under Islamic State control and an outspoken critic of al-Assad, was killed in the south-eastern Turkish province of Gaziantep in 2015.
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Police had detected him using surveillance footage from the Uskudar neighbourhood where the mother and daughter were killed, Dogan said.
Orouba Barakat, a former member of the opposition Syrian National Council, was known for her backing of the uprising against Syrian President Bashar al-Assad. But she was also critical of opposition groups.
Halla Barakat was a journalist with Orient News, a Syrian opposition broadcaster, Turkish media outlets had reported earlier.
This is not the first time opposition figures and journalists from neighbouring war-torn Syria have been targeted in Turkey.
Naji Jerf, a Syrian filmmaker who worked on exposing life under Islamic State control and an outspoken critic of al-Assad, was killed in the south-eastern Turkish province of Gaziantep in 2015.
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