On Friday, Rajapaksa appointed his elder brother as finance minister of the interim government, Reuters reported.
According to media reports, it is the first time in Sri Lankan history that siblings have held the two top political positions in the country.
Gotabaya and Mahinda Rajapaksas are members of one of the oldest and most powerful political clans in Sri Lanka. The latter served as president of the island nation from 2005 to 2015.
Sri Lanka held a presidential election last Saturday. Gotabaya Rajapaksa won with 52.25 percent of the vote. His key rival, ruling party candidate Sajith Premadasa, was supported by 42 percent of voters.
Gotabaya Rajapaksa previously served as the country's deputy defence minister. He rose to fame after participating in the defeat of Tamil separatists during the final stage of the civil war, which lasted from 1983 to 2009. Rajapaksa has pledged to focus on economic reform and improvements to the country's security system.