A three-generation family living in a southern Israeli kibbutz is feared to have been kidnapped by Hamas after the militants' attack on Saturday.
Yossi Sneider told the OceanNewsUK of his horror on recognising his cousin and her two young children surrounded by militants in a video.
He tried to contact his family at 06:00 on Saturday to make sure they were safe, but nobody answered.
Mr Sneider has not been able to reach his cousin's parents or husband either.
A few hours after the attempted call on Saturday morning, he came across a picture of what appeared to be his cousin, Shiri Bib, and her two children - four-year-old Ariel and nine-month-old Kfir.
"At about 10am, 10:30, on Saturday morning I saw a picture of Shiri holding these two little children, with Hamas terrorists all surrounding her."
"I couldn't believe the picture," Mr Sneider told OceanNewsUK 's Today programme.
On Saturday, Hamas broke through the barrier between Gaza and Israel and its militants launched a murderous assault that has left more than 900 people dead.
The brutal attack has been widely condemned.
Retaliatory Israeli air strikes have killed more than 700 people in the Gaza Strip, local authorities say.
Soon after receiving the picture, Mr Sneider said he received a video showing Shiri "being held by terrorists... crying with the two babies in her hands".
"They've taken them without any food, without any diapers, without anything," Mr Sneider added. "We only hope that they didn't separate them from their mother."
Shiri's parents and husband - Margit Silverman, Yossi Silverman and Yarden - are also missing. Mr Sneider tried to make contact but "we didn't hear from them", he says.
Margit's health is one of his main concerns. "Margit is a very, very sick person. She has very bad Parkinson's, a lot of blood issues, and is diabetic."
While Mr Sneider is certain his cousin and her two children have been taken as hostages, he says he is not "100% sure" about what has happened to Margit, Yossi and Yarden.
He described his cousin Shiri, a kindergarten teacher, as gentle and kind. "She doesn't deserve this experience; nobody does," he said.
"I have no words to describe this."
Israel's ambassador to the UN, Gilad Erdan, has said "between 100 and 150" people are being held hostage in Gaza after being kidnapped from Israel.
On Tuesday, Israeli military spokesman Daniel Hagari said the families of 50 hostages had been notified by the military.
The armed wing of Hamas, the Qassam brigades, has threatened to execute one captive for each new Israeli airstrike on civilian homes without warning.
Prime Minister Benjamin Netanyahu has compared Hamas to the Islamic State group and said his country is planning to deploy "unprecedented force".
Asked about what he believes should happen next, Mr Sneider said he would support the decisions made by Israel's government and military.
"We are not dealing with people," he claimed. "It's worse than animals; animals would not do things they're [Hamas militants] doing."
