Prosecutors said Khaw Kim-sun left the inflatable ball in the boot of a car where the gas leaked out and killed them in the case that baffled police in 2015, reports the BBC.
Khaw pleaded not guilty to two counts of murder.
A post-mortem examination showed his wife and eldest daughter had died from carbon monoxide poisoning.
The police found a deflated yoga ball in the back of the yellow Mini Cooper in which their bodies were discovered.
Prosecutors told Hong Kong's High Court that Khaw had deliberately plotted to kill his wife because he was having an affair and she would not grant him a divorce.
But they said it was unlikely he wanted to kill his daughter. He allegedly told his younger daughter to stay at home to complete homework.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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