After discovering pornographic drawings in an exercise book belonging to her 11-year-old son, she confronted him, expecting him to apologise for what he did.
To the woman's shock, he broke down and told her that he had been sexually abused by his stepfather since he was nine years old.
Yesterday, the 35-year-old man, who is self-employed, was sentenced to seven years' jail and ordered to be given three strokes of the cane after he pleaded guilty to one charge of sexually abusing his stepson.
Six other charges relating to the same victim were taken into consideration.
The man cannot be named due to a gag order to protect the victim's identity.
Deputy Public Prosecutor Nicholas Lai told the court that the woman had entered into a relationship with the man soon after her divorce in 2009.
In 2010, he moved into her Jurong West flat, where she lives with her son. The couple registered their marriage in July 2014.
But barely half a year later, she discovered that her new husband had been sexually abusing her son.
One incident took place between 7am and 8am on Jan 17, 2015. The woman was sleeping when the man and the boy entered the master bedroom. They both got onto the queen size bed with the man lying between the boy and his mother, said DPP Lai. After placing a blanket over them, the man started touching the boy's private parts. Then he forced the boy to perform oral sex on him.
DPP Lai said the boy did not tell anyone about the incident because he was afraid of angering his stepfather. "He also did not know how to tell anyone about the said incident as the accused had been very nice to his family, and had also told the victim not to tell anyone about the encounter," DPP Lai added.
The abuse came to light when the woman made a police report after her son told her what the man had been doing to him.
In their mitigation plea, the man's lawyers, Mr Josephus Tan and Mr Cory Wong, said he was "very remorseful".
The man himself was sexually abused in his youth, which led him to perceive such sexual acts as normal, they added.
For sexually penetrating a minor under 14 years old, the man could have been jailed up to 20 years and fined or caned.