One of the two brothers died in a suicide bombing in April, the magazine 'Dabiq' reports.
Known within the organisation as Abu Mus'ab al-Almani - father of Mus'ab, the German - the 25-year-old is reported to have driven a truck containing seven tonnes of explosives into the headquarters of the 4th Regiment of the Iraqi army in Wilayat Shamal, northwest of Baghdad.
The magazine reports that the subsequent explosion completely destroyed the command centre building.
It also claims that al-Almani had fought for the German 'crusader army' in Afghanistan before joining Isis.
The other brother also died in a suicide attack, reports the magazine.
The brothers converted to Islam recently, reports WAZ. It is unclear when they travelled to Syria or whether the German authorities had been keeping tabs on them.
In March, Interior Minister Thomas de Maizière said that around 650 Germans had already travelled to the war zone in Iraq and Syria with the intention of fighting.