The sister of a woman jailed in Egypt for alleged ties to the outlawed Muslim Brotherhood is urging the Islamic State group not to kill a Croatian hostage.

The group had earlier said it would do so in the coming hours if the Egyptian government does not release jailed "Muslim women."

Doaa el-Taweel, sister of jailed photographer and activist Esraa el-Taweel, said on Friday that her sister is innocent but that "her release should not come at the expense of another innocent person."

Extremists of the Islamic State released a video on Wednesday threatening to kill the Croatian hostage in 48 hours if authorities failed to release "Muslim women" held in Egyptian jails — a reference to female Islamists detained in the government's broad crackdown on Brotherhood supporters.