Two teenage sisters from London convicted of killing a pensioner who had befriended them were given long prison sentences yesterday.

Kellie Lyons, 17, was given 10 years for the manslaughter of Rose Mackenzie, 87, and eight years concurrently for robbery. Her sister Jean, 19, was given eight years and six years concurrently for robbery.

At the Old Bailey, Judge Coombe said: "This was as mean and despicable an offence as can be imagined." The killing was so seriously that the sisters should be kept on licence for 11 years after their release, he said.

Mrs Mackenzie died a week after being attacked at her home in Manor House, north London, in February. The sisters, wearing balaclavas, had broken in, attacked her and stolen £800 from a hiding place in the bathroom.

During the trial, the sisters, who lived in the same block of flats as the pensioner, blamed each other for the death. Jean received a lesser sentence because she admitted her part in the robbery.

A robbery on an elderly person in their home was one of the "most heinous" crimes, the judge said. "What makes this crime even graver is that Rose Mackenzie had been a kind lady who befriended these two. She had lent the family money from time to time, there were birthday cards and sweets for what she took to be friendly, young children."

The judge said he suspected the jury had cleared the sisters of murder because they suspected that one of the girls was guilty of murder – but did not know which one.

Ann Curnow QC, for the prosecution, said the pair had left the widow with multiple rib fractures and boasted about what they had done. The sisters asked friends to get rid of the clothing they had worn during the crime and a plastic bag containing documents. The friends contacted police after hearing of the death.