Abdullahi Ahmed Jama Farah, 20, from Manchester, was sentenced to seven years at a young offenders institution.
He created a UK "hub of communication" for like-minded extremists in 2013, the Old Bailey heard.
Jama Farah is related to Zhara and Salma Halane, who are believed to have married IS fighters in Syria.
The radicalised student was studying for his A-levels when he created the hub from his mother's home in South Grove, Longsight.
He was arrested in 2014 and following a trial, found guilty of preparing for terrorist acts after attempting to facilitate Nur Hassan, 19, to travel to Syria to fight.
Sentencing him at the Old Bailey, Judge Michael Topolski told him that the extent of his radicalisation was "considerable".
He said:  "I am satisfied that what motivated you to assist was the very same set of extremist beliefs that motivated your friends to travel and train and fight and if necessary, to die."
The judge also highlighted messages in which the defendant referred to beheadings as "lick some heads off" and asking his friends if they were "smacking (killing) guys".
As well as helping Nur Hassan travel to Syria, the court heard he had been in contact with friends abroad via messaging service Whatsapp and social media.
The jury convicted Farah for helping Hassan and for his communication with Khalil Raoufi, who was killed in combat.
The Old Bailey heard that Jama Farah, who is Danish and of Somali origin, was in contact with four friends abroad.
Two are believed to have been killed in fighting and another badly injured in fighting.
His cousin Ahmed Ibrahim Halane, known as Pie, from Manchester, went to Somalia in September 2013, where it is thought he went to join terror group Al Shabaab.
He was sentenced to a further three years on extended licence.
A cousin of sisters dubbed the "teenage terror twins" after they joined the so-called Islamic State has been detained for helping a friend travel to Syria.