Alan Sugar
Lord Sugar has been presenting the Apprentice for more than a decadeGetty/Staff

The half-brother of Yasmina Siadatan, the winner of the 2009 edition of the Apprentice, has been in Syria for a year and a half, fighting for Islamic State (Isis). Isaiah Siadatan, 24, left his home in Walsall in order to join Isis (Daesh) in the summer of 2014 along with two friends, leaving his young family behind.

He soon tried to force his wife, Kerry Thomason, 23, to join him in the hopes of indoctrinating his children, a court heard this week. She was arrested as she tried to board a flight to Turkey with them last October.

The revelations came to light at the trial of four Muslim extremists, including Thomason, at the Old Bailey. They were found of helping others travel to Syria to commit acts of terrorism, and are due to be sentenced to jail time.

Yasmina, 34, is Isaiah's half-sister. Their father Medi Siadatan, 62, an Iranian who in the 1990s unsuccessfully attempted to get Islamic polygamy – which allows a man to have up to four wives – recognised under English laws.

Yasmina won a £100,000-a-year job on the fifth series of the Apprentice in 2009 with presenter Alan Sugar, but did not return after going on maternity leave. She still regularly appears on Sky News, reviewing current affairs, where she has spoken out against IS and Islamic extremism.

A spokesman for Yasmina told the Mail Online that she "condemned her brother's actions in Syria, and revealed she had not had any contact with him for more than five years".

The spokesman added: "She wishes to make it clear that she condemns these alleged actions and political views in the strongest possible terms."

Isaiah's current whereabouts are unknown, and unconfirmed reports suggest he might have been killed in action.

The court heard that he drove his school friend, Jacob Petty, the son of a Church of England vicar, to the airport two years ago so he could join IS. He joined Petty in June 2014, leaving his family behind in Walsall.

A month after his wife and children had their passports seized at Birmingham airport, he sent his spouse an email threatening to have her and her parents killed if she did not join him in Syria. He wrote: "If you don't bring my kids to the Islamic State I will send someone to kill you and I will send someone to kill your mum and dad.

"You have two weeks from today. Look I love you but if you think I will let you bring up my kids in a kafir country you're mistaken."

Thomason was convicted alongside Alex Nash, 22, a Muslim convert, Aymen Shaukat, 27, and Lorna Moore, 33 – the maths teacher wife of Sajid Aslam, who had also travelled to Syria with Isaiah.