Annabelle, 21, died in a Sydney hospital on Friday. Jessica, 28, who appeared on TV soap Home and Away in 2016, remains in a critical condition.
The family were caught up in a two-car highway collision on Boxing Day near Ulladulla, New South Wales.
Their parents and the driver of the other car died at the scene.
The horror crash has made headlines across Australia, with police calling it "an absolute tragedy".
Annabelle's boyfriend had shared a Snapchat picture of them both shortly after the crash, asking for prayers and support.