A 25-year-old man sexually exploited his much younger siblings for his own perverse gratification instead of caring for and protecting them, a district judge noted.

So Judge Shaiffudin Saruwan yesterday jailed the man a total of 7½ years on three charges - sexual assault by penetration, and two counts of committing an indecent act with a child.

The court heard last month that one victim was a boy, who was abused when he was between seven and 10 years old. The other was his sister, who was eight at the time of the offence against her.

The offences came to light in mid-2013, when the victims' father discovered that the boy, aged 11 then, had made his eight-year-old sister perform oral sex on him.

The boy said he had been sexually abused by both the father's 53-year-old helper - who was living with the family between December 2009 and January 2010 - and the accused, and that he had learnt it from them.

He also revealed that there were several occasions when the accused made him and his sister use their legs to rub his private parts.

The helper was 56 when he was jailed six years in October last year for sexually abusing the boy, with two other charges taken into consideration.

The court heard that in 2012, the current accused had taken his stepbrother to a multi-storey carpark opposite Block 29, Havelock Road, where he committed the sexual offence at a staircase landing.

The boy, then in Primary 4, did not tell anyone as he was warned by the accused that there would be a big family problem if he did so.

On June 20, 2013, the accused covered the boy's mouth while they were alone at home and took him to a room to commit the indecent act. Some time during that month, the accused did the same with his stepsister.

Defence lawyer G. Dinagaran said his client, who has mild intellectual disability, was a slow learner, failed every year during primary school, and could not cope when he was referred to a special school.

Judge Shaiffudin, who took six other charges into consideration, said the victims' ages rendered them very vulnerable and defenceless.

In the case of the boy, he said his vulnerability was made more prominent because of his low average intelligence and he was the victim of an earlier episode of a similar sexual assault by a stranger.

The accused could have been jailed up to 20 years, fined or caned for sexual penetration of a minor. He could have been fined up to $4,000 and jailed for up to four years on each charge under the Children and Young Persons Act.