Omobowale told Justice Sedoten Ogunsanya that the incident occurred at their Maryland, Lagos apartment in 2014.
She was 11 at the time, she explained, and about to begin Junior Secondary School (JSS) 1, while her brother, Tunde, was 13.
Omobowale, now 17, testified during the trial of her mother, Margaret Olufunmilayo Odunlade, her stepfather, Folorunsho Emmanuel and his mother, Funke Maxwell, for the alleged offence.
Odunlade, Emmanuel and Maxwell were arraigned by the Lagos State Government in 2014, on a two-count charge of manslaughter and misconduct with regard to a corpse.
Prosecuting counsel, Adebayo Haroun, said the offences were committed at about 6am, on August 2, 2014, at 17, Emmanuel Street, Maryland, Ikeja, Lagos.
He said the defendants unlawfully killed Tunde Sunmola by beating him and not taking him to the hospital for treatment of injuries he sustained on his leg.
Odunlade was also accused of dumping Tunde’s body in a canal.
According to the prosecutor, the offences contravened sections 222 and 163 of the Criminal Law of Lagos State, 2011.
Omobowale, led in evidence by Haroun, testified as the third prosecution witness.
The witness said at the time, she lived with her mom, stepdad and his mom, while Tunde lived in Ijebu Ode with their grandmother, Mrs. Arinola Sunmola.
She said Tunde was eager to see her and their mum, so their grandmother persuaded his mother to come along to Ijebu with her (Sunmola) and take him to Lagos on August 1.
According to Omobowale, when they got to Lagos, “My mother asked Tunde to start moving ahead of us and when I asked her why, she said she didn’t want people in the area to know that Tunde was her son.
That night when my mom’s husband came back from work, he asked my mum, ‘who is this boy?’
“My mom said: ‘He is one of my sons.’
“Her husband said: ‘Did you tell me you had another son outside?’
“My mum said it was a mistake.
“So, they argued and he asked why she didn’t tell him about my brother. She could not say anything,
“That night, after we slept, I heard, my mum’s husband and his mother beating my brother. My mum later carried my brother on her back and I woke up and asked her, ‘where are you taking him?’ She said she was taking him back to Ijebu. I told her we were going together but she refused.
“When she returned, I asked her where my brother was. She said she had returned him to Ijebu.”
The witness said the following day, a relation, ‘Aunty Yemi’, told her that her brother was dead.
The witness said Omobowale later informed her that Tunde was beaten to death by her mother’s new husband and his mother.
Following the conclusion of testimonies, Justice Sedoten Ogunsanya adjourned further proceedings till June 26.