The allegations were detailed in Melbourne Magistrates Court on Thursday as Zeinab Ahmad, 31, applied for bail, a month after she was charged with slavery offences.
Australian Federal Police allege Ahmad left Melbourne to go to Turkey with her husband Dawod in November 2014.
She told migration officers her plan was to stay there for seven months but it's alleged the couple instead moved to Syria in January 2015.
Ahmad's husband Dawod became an IS member but he died in a Syrian drone strike in May 2016, Detective Senior Constable Marc Clendenning told the court.
After his death, it's alleged Ahmad made social media posts saying Dawod had lived out his dream of becoming a martyr.
It is also alleged Ahmad made a post calling on Allah to "destroy the United States and its allies".
Police say Ahmad was living in the family home in Syria in 2017 when her father Mohammad bought a teenage girl as a slave for $US10,000.
It's alleged the girl was repeatedly raped and beaten by Mohammad, including an incident where she was hit and dragged down two flights of stairs by the hair.
The girl told police she had screamed loudly during the incidents so Ahmad and the other family members would have known what was happening, the detective said.
It is not alleged Ahmad assaulted the girl but the teen claims she was "treated very badly" by the woman.
The girl allegedly lived with the Ahmad family for 16 months before she was sold in November 2018.
The teen was sold a further seven times before she was freed from slavery.
Ahmad and her mother Kawsar, 52, were among a group of alleged "ISIS brides" and their children who returned to Australia in May.
She was arrested at Melbourne Airport and charged with offences of enslavement and using a slave.
Human rights activist Robert Van Aalst, who helped the women and children return to Australia from Syria, attended court on Thursday along with supporters of Ahmad.
In opposing bail on Thursday, Sen Const Clendenning told the court Ahmad was an unacceptable risk of endangering the community.
She had married another two IS members and was still married to an Egyptian-born fighter whose location was unknown, the detective told the court.
Ahmad had never explicitly renounced IS and she pursued living under the terror group in Syria, Sen Const Clendenning said.
He said she had been employed by IS and encouraged violence against individuals and governments.
The bail application before Chief Magistrate Lisa Hannan will continue on Friday.
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