The brother of Nigerian militant Henry Okah has been charged with kidnapping nine foreign oil workers this year.
Charles Okah and three other men denied taking hostages in the oil-producing Niger Delta between August and October when they appeared in court.
Charles Okah was charged last month over the Independence Day attacks in Abuja. His brother remains in custody in South Africa, accused of masterminding the car-bombings.
The oil workers were freed this month.
Nineteen oil workers were rescued in an army operation.
The authorities said 63 people had been arrested.
Charles Okah and the other men were accused of kidnapping four Indians, three French citizens and two Russians.
Most of Nigeria's oil militants have laid down their weapons as part of a government amnesty but one faction is believed to be behind a recent upsurge of violence.
This began with the attacks on the capital Abuja, which killed 12 people on 1 October, as Nigeria was celebrating 50 years of independence from the UK.
The Movement for the Emancipation of the Niger Delta (Mend) says it is fighting so that more of Nigeria's massive oil wealth is used to benefit the Niger Delta area which produces the oil.
But criminal gangs have taken advantage of the region's instability to make money from ransoms paid by oil companies, and by stealing oil.