Residents found the body of the daughter of the two slain doctors on Thursday evening, said the source at the Ibn Sina hospital in the city of Sirte, without giving the cause of death.
She said the girl was 13 years old. Initial reports had given her age as 18.
After murdering her parents in their home on Tuesday, the attackers took the girl but left behind two sisters, local council chairman Yussef Tebeiqa said on the day of the incident.
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Tebeiqa said the attack in Sirte, home town of slain dictator Moamer Kadhafi, might have been motivated by religion as money and jewellery were not taken.
Sirte, 500 kilometres (310 miles) east of Tripoli, is in the hands of Islamist militias including Ansar al-Sharia, which the UN Security Council last month added to its terror list over links to Al-Qaeda and for running Islamic State group training camps.
Several Coptic Christian Egyptians have been killed in Libya in recent years. In February, the bodies of seven Egyptian Christians who had been shot were found near the second city of Benghazi.
Thousands of Egyptians work in Libya, mainly in the construction and craft sectors.