A 16-year-old girl has been jailed and her younger sister given a youth rehabilitation order for their part in a plot to kill their 89-year-old grandfather so they could steal his money.

The elderly man, who suffered from dementia, was attacked with bricks at his bungalow in a village near Winchester, Hampshire.

In the weeks before, some of those involved in the plot had researched how to kill him on the internet using Google searches such as “1,000 ways to die”, “poisonous toadstools” and “easiest way to kill an old person”.

Last month, the elderly victim’s adopted daughter, 49, was jailed for 17 years and her son, 19, was given an indeterminate sentence in a young offenders’ institution after being found guilty of conspiracy to murder.

The woman’s older daughter, 16, was also found guilty of conspiracy to murder and was given a 26-month youth detention order at Winchester crown court. her younger sister, 14, was given a two-year youth rehabilitation order after she was convicted of wounding with intent, but was acquitted of the conspiracy charge.

A third girl, aged 17 – the 19-year-old son’s girlfriend – was also found guilty of the conspiracy charge and sentenced to three years’ youth detention.

The girls, sitting with social workers, sobbed as the sentences were handed down. Sentencing the two older girls, Mr Justice Foskett said: “The essence of the offence of which you were convicted is that you were prepared to contemplate the death of another individual. I cannot avoid a custodial sentence in your two cases.”

Speaking to the younger sister, he said: “Despite your physical appearance, you are still very young and immature. I do not think the public interest calls for a custodial sentence in your case.”

The judge described the three girls as “vulnerable” and under the influence of the mother, adding that they had acted out of fear of her.

Sentencing the two adults last month, Foskett called the attack “despicable and inhuman”. He said the plot and the attempts to kill the pensioner, who lived with his wife, “will defy belief in the minds of any right-thinking person”.

The family, who cannot be named for legal reasons, had launched a campaign to try to scare the OAP to death by smashing a window at his home and cutting the fuel line of his car to try to make it explode.

During the six-week trial the jury was told the man was lured outside his home by his daughter pretending to have fallen over. He was knocked to the ground by her son and hit with bricks by the two young girls.

The prosecution said the intention was to kill him for his money even though he had generously given cash for cars and horses for the family, which had been squandered.

The man survived with cuts and bruises and was able to tell paramedics, who were called by his daughter, he had been hit. He is now in a residential home with his wife.

The court heard that the daughter stopped the attack and tried to tell ambulance staff her father had fallen, but the family was arrested.

<a href="http://www.guardian.co.uk/uk/2011/sep/29/sisters-sentenced-plot-kill-grandfather?newsfeed=truetag:news.google.com,2005:cluster=http://www.guardian.co.uk/uk/2011/sep/29/sisters-sentenced-plot-kill-grandfather?newsfeed=trueThu, 29 Sep 2011 14:26:41 GMT 00:00″>Teenage sisters sentenced over plot to kill their grandfather, 89