Chloe Craig, who was a friend of Shayla, often stayed overnight at the house.
She soon became aware Sharon was disabled and needed caring for.
Prosecutor Ashley Edwards QC asked the teenager about visits to the house in 2016.
Chloe, 19, claimed she spotted Sharon with a black eye but the mother said she got it from pulling a plug from a sink.
The witness recalled then having a conversation with Shayla about the injury.
She told the court Shayla "thought" Lynnette was responsible.
It was claimed Lynnette was "not bothered" that it may be known she allegedly hit Sharon.
The advocate depute asked Chloe what Lynnette's "demeanour" was like at the time.
She replied: "She was laughing about it."
Miss Edwards: "As well as laughing, was she giving any other impression?"
Chloe: "No, she just seemed to find it funny."
The teenager recalled another occasion when Sharon was in bed and she started crying.
Chloe told the jury: "She said she did not want to be there and did not want Lynnette near her."
A charity worker later told the trial he went to collect a couch from the Greenop house - and thought he spotted a dead body.
Mark Grant went with colleagues to the property in August 2016.
He told the court that when he pushed a bedroom door open and he noticed someone lying on the floor.
[...]
Mr Grant helped collect the suite before revealing what he had seen to his supervisor.
The witness recalled: "I got in the van and said: 'It looks like someone is up there dead'.
"It was a nervous giggle at the time.
"The supervisor - I don't think - believed me."