Officers detained the men, aged between 25 and 31, in an early morning raid on their home in the northeastern city of Lleida, a police spokesman said.
The brothers operated several social media accounts which they used to upload videos praising group executions carried out by Islamic State, the Taliban and other Pakistani extremist groups, police said in a statement.
Police said the brothers had gathered a large group of followers in their attempts to spread jihadist ideology.
The raid comes just three days after a gun and suicide bomb spree at Istanbul's Ataturk airport that killed 44 people and which Turkish officials said appeared to have been carried out by the Islamic State group.
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