'No sister would want their brother to go to Syria'
There are concerns many men are going to Syria to fight in the country's civil war.
Police think up to 20 people from the UK have died there and they are now calling on Muslim women from the UK to do more to stop the men they know from getting involved.
Abdullah Deghayes left the UK to fight in the conflict, along with his two brothers Amer and Jaffar.
The 18-year-old, who was from Brighton, was killed a few weeks ago.
The foreign secretary has urged British people not to fight in Syria and police say they face arrest on their return.
Abdullah's sister Aisha, 21, spoke exclusively with Newsbeat and said she had no idea what her brothers had planned.
"I didn't have any idea," she said. "He didn't talk to anyone about it. He never brought up Syria.
"We never thought he knew what was going on in Syria because he was too busy with his friends and going out."
She said she found out her brother had died through a picture in the news.
"I was shocked, I didn't want to believe it," she said. "I didn't want to believe it was him, but it looked a lot like him so it was very hard."
If she had known they were going, Aisha says she would have tried to stop them.
"No mum or sister would want their brother or father to go to Syria and they would do their maximum to stop them from going," she said.
Aisha said she had spoken with all three of her brothers after they left the family home.
"I did speak to them on Facebook and on the phone. I did tell them please come back, please come back," she said.
"I'd get into arguments with them saying, 'Mum is stressed' or, 'Mum is ill.' I tried everything to be honest and I think every mum and sister would do the same."
Her brothers Amer and Jaffar are still fighting in Syria but Aisha said she could not force them to come home.
"I really hope the Syrian war ends to be honest," she added.
"Amer and Jeffar are only a few of many younger boys who go out there and get stuck there.
"Abdullah is only one out of many who have died over there."
Aisha says life is now a constant worry for her and the rest of her family.
"It's hard to sleep at night when you know your brother is in the middle of a war zone," she said.
"You don't know what he's doing and he didn't even tell you what he's doing, he just said things to comfort you."
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