The killing comes just two days after Tariq al-Hashemi, a leading Sunni official, was named as vice-president, and a day after he called for Iraq's insurgency to be put down with force.
Meysoun al-Hashemi and her driver were leaving her home in Baghdad's al-Alam neighbourhood when four gunmen in a BMW shot them both in a drive-by attack.
The killing comes after one of Mr Hashemi’s brothers was killed on April 13.
Ms Hashemi was married with children, and was the head of the women's affairs department in the Iraqi Islamic Party, the biggest Sunni Muslim political faction, which is headed by her brother.
The brother of another leading Sunni politician, Saleh al-Mutlak, was found dead this month after he was kidnapped.
This comes as at least four people were killed in Nasiriyah, in southern Iraq, when a blast hit a military convoy.
Reports say three Italian soldiers and one Romanian soldier were killed in the blast.
More than 2,000 Italian troops are stationed in Nasiriyah.
The latest violence comes as US Secretary of State Condoleezza Rice and US Defence Secretary Donald Rumsfeld were visiting Baghdad to meet with officials in the new Iraqi government.
Unity government
Mr Hashemi's Iraqi Islamic Party is the largest in the main Sunni political bloc and was the first Sunni Islamist party to join the political process, which US and Iraqi officials hope will defuse the Sunni insurgency.
He made his first public appearance as vice president on Wednesday at a news conference with President Jalal Talabani, a Kurd, and Shi'ite Adel Abdul Mahdi, the other vice president in the new administration.
They vowed to press ahead with efforts to form a national unity government they hope can avert a sectarian civil war.
Nouri al-Maliki, a Shi'ite hardliner recently named as Iraq's prime minister-designate, is progressing with efforts to form a new national unity government, aimed at ending sectarian violence.
He has 30 days to assemble a cabinet from divided Shi'te, Sunni and Kurdish parties, with the most contentious positions in ministries controlling the security forces.