Charles Okah, the brother of Nigerian militant Henry Okah, has been arrested in Lagos on suspicion of aiding two deadly car bombings in the capital, Abuja, security sources say.
The attacks on 1 October during celebrations of the 50th anniversary of independence killed at least 12 people.
Henry Okah was detained in South Africa after the blasts and is facing terrorism charges.
He formerly led militants Mend, who say oil wealth must be fairly distributed.
The group he now leads is believed to have split from the main Mend - the Movement for the Emancipation of the Niger Delta - which signed an agreement with the government last year to end years of unrest in the oil-producing Niger Delta region.
One security source told Associated Press that Charles Okah was detained during a raid on his home in Lagos on Saturday.
"[He] has been mentioned... by suspects with us as a source of funds for the blast. He is with us in Abuja," the source said.
Charles Okah is also suspected of distributing information on behalf on Mend.
Charles Okah used the pseudonym Jomo Gbomo "to threaten and cause confusion" in emailed statements, the sources said.
An email warning of another bomb attack in Abuja was sent on Saturday and signed Jomo Gbomo, Reuters news agency quoted the sources as saying.
Henry Okah left Nigeria for South Africa after being freed from prison in July 2009.
He has denied links to the Abuja blasts but prosecutors say he was the mastermind.
Mend had issued a threat saying it intended to target the independence event in Abuja shortly before the two bombs went off, about five minutes apart.