abdulkadir zeyat , 19 , had been out at work in the north-western city of istanbul .
turkish authorities believe the family , which included siblings as young as three-years-old , died of food poisoning .
kemal zeyat , 19 , and six of their children -lrb- one of whom are pictured and his parents were dead .
mr and mrs zeyat , and six children were all discovered at their house in the village of alintepe .