Salim Wakil, 25, arranged the transfer of 3,000 dollars (about £2,300) via Western Union for his sister Sumaiyyah – who married Portsmouth Isis fighter Mehdi Hassan – in February last year, despite repeated warnings from police.
While in Syria, Sumaiyyah had married 19-year-old Abu Dujana - real name Mehdi Hassan - from Portsmouth, who was killed fighting in the autumn of 2014.
Wakil used a friend's name in the Western Union transaction in a 'deliberate attempt by the defendant to conceal his actions, demonstrating he knew perfectly well what he was doing was wrong', the Old Bailey heard.
Wakil claimed he only sent the funds to help his younger sibling return to family in Britain, even though she had married and given birth to a daughter in Raqqa.
Emergency crews battling to save someone’s life on Southsea seafront
Police update on probe into death of ‘Wiggy’ Symes after fatal dog attack in Fareham
Thunderstorms in Portsmouth yellow weather warning: The Met Office forecast for the next three days in city, Fareham, Gosport, Havant and Waterlooville and Hampshire
Southsea seafront incident: Police update after man found 'unresponsive'
River Ems crisis: Westbourne villagers plead with Portsmouth Water to introduce a hosepipe ban
But he was jailed for 30 months on Friday by Judge Rebecca Poulet QC after he was found guilty of entering into a funding arrangement by a jury.
After he was convicted last month the judge said she had 'no doubt this defendant was naive' and was not supportive of IS himself.
She said he 'foolishly and wrongly' took the risk of sending money because he was 'genuinely feeling concerned for the safety of his sister'.
Wakil had lived with his parents and nine younger siblings at the family home in Fleet, Hampshire, including Sumaiyyah, who is now 21, the court heard.
In August 2014, aged just 16, Sumaiyyah left home and travelled to Syria, leaving behind a letter explaining her reasons for joining IS and asking her family not to tell police.
Police did not find out until the following year, and the Prevent deradicalisation programme was called in.
An investigation revealed the teenager had maintained contact with her family via Skype calls and WhatsApp as well as other text communications.
During the chats, Sumaiyyah described wanting to become a martyr and talked of her husband and pregnancy, jurors heard.
But Wakil repeatedly encouraged his sister to come back to Britain.
When she asked to see photographs of her younger siblings, Wakil replied: 'U come see them thanks.'
Detective Chief Superintendent Kath Barnes, head of Counter Terrorism Policing South East (CTPSE), said: 'Salim Wakil was manipulated by his sister into sending money to her, which could very easily be used for terrorism purposes.
'The law intends to cut off funding to terrorist groups and to stop money falling into the hands of people who may use it for terrorist purposes, and today's sentence reflects that.
'By making the decision to send money and ignoring the advice of the police, Salim Wakil broke the law.
'The law applies equally to everyone, regardless of their motives, and is here to stop the funding of terrorist organisations and individuals.
'No-one has the permission to take the law into their own hands, no matter how emotional the reasoning is for doing so.'