A Hong Kong court on Monday heard a young boy recount years of abuse at the hands of his family, culminating with his father – who stands accused of murder – throwing his five-year-old sister at the ceiling 18 times as punishment for her “walking slowly” on the night before she died.

Speaking in a pair of videos made the day after his sister’s death, the girl’s surviving eight-year-old brother also told investigators that she had been made to sleep on the floor without a jacket or a sleeping bag that January night, which he believed was the other reason for her collapse the following morning in 2018.

The High Court heard the girl died the same day of septicaemia – a serious infection in the blood – which prosecutors said was indirectly caused by her assault, ill-treatment and neglect at the hands of her 29-year-old father and 30-year-old stepmother, also accused of murder, between August 10, 2017 and January 6, 2018.

The couple has denied the murder charge, but have admitted child cruelty towards the deceased five-year-old and her brother during the same period. The children’s 56-year-old step-grandmother has denied all four counts of child cruelty she was charged with.

None of the defendants or their relatives can be named following a gag order from Mr Justice Albert Wong Sung-hau aimed at protecting the identity of the children.

Prosecutors on Monday played taped interviews with the young boy, identified only as X, that were conducted a day after the death of his sister, Z, on January 6, 2018. The siblings also had a stepsister, Y, from their stepmother’s previous marriage.

The boy recalled watching his father throw Z upwards 18 times on the night before her death, with her head “gently” hitting the ceiling – an act referred to among the family as “flying high”, which prosecutors characterised as a form of punishment disguised as a “game”. Their stepmother was also said to have joined in with another so-called game, swinging the girl’s body back and forth in an attempt to scare her and teach her a lesson for being slow and disobedient.

Z was crying and screaming during the incident, and appeared very scared, X said.

That night was also his first experience with “flying high”, which he allegedly “played” at his stepmother’s suggestion.

“It was very horrifying,” he said, recalling how his father had grabbed him by the waist and thrown him. “When I was in the air, it felt like I was falling from an aeroplane.”

The sibling said Z had been physically punished from the time she was three, with their parents hitting her with a “very thick and long” rattan cane 30 to 50 times per session whenever she was “naughty”, which X said was just about every other day.

When asked how he knew the number of hits, the boy replied: “I’ve experienced it, too.”

X, who confirmed at the start of the interviews that he knew what lies were, said he had been subjected to more than 100 beatings from each parent, and that his father would also punch him, slap him with the back of a slipper and poke his chest with a pair of scissors.

X said he would try to hold back tears “because boys were not supposed to cry”, and his father did not like it.

“But I couldn’t control it,” he said.

He did not, however, try to resist the alleged abuse, even when it was “very painful”.

“Because I deserve it,” he explained to the interviewing social worker. “I did something wrong. I told lies. I was dishonest.”

Offering an example, he said he had lied about chatting during class, claiming that he was only responding to other kids when he was the one who initiated conversation.

X said the beatings would leave him and Z with injuries, which their parents would treat and disinfect at home. He also said he once overheard his parents talking about the foul-smelling pus emanating from wounds on his sister’s legs, but said he did not see those injuries.

He added that he had “a feeling” that his family did not want the others to see the injuries because he was told he would have to behave if he wanted to go to school.

His interviews also revealed other kinds of punishment, like being deprived of a bed and made to sleep in a sleeping bag or on the floor.

He also recalled sharing leftovers with Z after they were forbidden from eating with their parents and Y as punishment for misbehaving, and were instead made to stand facing the wall or to walk in circles. X also recalled him and his sister going without food – at times for days – when they could not meet their parents’ demands.

“I dreamt that I ate,” he said. “So I didn’t feel hungry.”

X said his parents got “very angry” when they learned he had spoken to his teacher and a social worker after they found him limping at school and made enquiries, but that they later “forgave” him after he apologised.

His video testimony continues on Tuesday.