The pair was arrested at an address in the Small Heath area of Birmingham, some 195 km northwest of London, under the Terrorism Act, West Midlands Police said.
They were arrested on suspicion of possessing documents likely to be of use to someone committing or preparing an act of terrorism, the BBC reported.
The police did not name the pair or their nationality.
Police said the arrests had been pre-planned and not made in response to any threat to public safety.
Computers and other electronic devices have been seized and will be forensically examined.
Police said the investigation was likely to take some time due to the number of files which will have to be assessed.
London: A 23-year-old man and his 18-year- old sister were arrested on Thursday in Birmingham on suspicion of possessing documents likely to be of use to someone preparing for an act of terrorism.
First Published: Thursday, October 04, 2012, 19:10