[2] Kostić was born in Mostar in present-day Bosnia and Herzegovina to a Bosnian mother and a Croat-Serb father, and they were raised there.
[6] They were installed as MP on 6 December 2023 as the first non-binary member in Dutch parliamentary history, and they are their party's spokesperson for environment, agriculture, the interior, infrastructure, housing, education, culture, social affairs, and healthcare.
[1][3][7] When the right-wing Schoof cabinet was formed, Kostić asked ministers-designate of the populist Party for Freedom (PVV) about their views on the Great Replacement conspiracy theory in response to previous controversial statements.
A motion filed by Kostić urging the government to meet nature conservation commitments passed the House with the support of two coalition parties.
[5] Later, alongside Dion Graus (PVV), Kostić called for a ban on the trade and ownership of animals prohibited from breeding, such as pugs and Sphynx cats.