Hove Crown Court in East Sussex was told that Sheila Bowler, a music teacher, collected Florence Jackson, 89, from a nursing home before driving her to the river Brede and pushing her in. Anthony Glass QC, for the prosecution, said Mrs Bowler killed her aunt because the home's fees of more than pounds 1,000 a month were eating into her inheritance.
Jo Dobson, head of the Greyfriars nursing home in Winchelsea, East Sussex, where Mrs Jackson was a resident, said: 'She was a delight to care for and had a lovely sense of humour despite being frail.'
Mr Glass said Mrs Jackson, who owned a small flat in Rye, East Sussex, was put into an old people's home six months before her death.
'Her flat had to be sold and the weekly cost of keeping her in the home would come from the sale proceeds, thereby reducing what Mrs Bowler would inherit.'
He added: 'The defendant is the sole beneficiary of Florence Jackson's estate. In other words she was to inherit what little Florence Jackson had to leave. It was greed and money, the age-old motives, that set this defendant on the cruel and premeditated murder. A couple of blows and a shove ended the life of Florence Jackson. They were the acts of calm, cool and cruel deliberation.'
Mr Glass said that after the murder on 13 May last year, Mrs Bowler drove her car back on to the main road and let down a tyre so that she had an excuse to leave the vehicle unattended. She had rung for a breakdown service, then said that Mrs Jackson was missing when she returned to the car.
Mr Glass said that Mrs Bowler's explanation as to what had happened would have meant that the killer must have been a 'wandering homicidal maniac'.
Later examination of Mrs Bowler's car showed that the tyre had not had a puncture.
Mrs Bowler, of Rye, East Sussex, denies murder.
The trial continues today.