Dawn McKenzie, 34, was stabbed to death by David McCourt after they argued about his behaviour and rule-breaking.
McCourt was 13 when he stabbed Miss McKenzie ten times on the head and body in 2011 at the home she shared with husband Bryan in Hamilton, Lanarkshire.
McCourt – who has now turned 18 – claimed “voices in his head” told him to do it.
He was convicted of culpable homicide and served four years of a seven-year sentence at St Mary’s Kenmure unit at Bishopbriggs, near Glasgow.
He was moved to a halfway house in February before being released under a supervision order.
It has now emerged that her family are seeking damages from Foster Care Associates Ltd, the agency who placed him in their care.
The action at Hamilton Sheriff Court had originally included a claim against Glasgow City Council, but that has now been dropped by the family.
The case had been due to call before a sheriff last month but has now been postponed.
The family are seeking £500,000 for Mr McKenzie in compensation for Dawn’s death and £200,000 for her mother, Ray Byrne.
Both claims seek damages for loss and injury as a result of alleged negligence.
Mr McKenzie and Mrs Byrne are being represented by the Lanarkshire Law Practice.
A spokesman for the firm confirmed that the case was ongoing but would not comment further. Foster Care Associates Ltd have denied liability but declined to comment.
Mr McKenzie told a fatal accident inquiry into the killing that they had not been given enough information about the boy and had been let down by social workers.
Sheriff David Bicket ruled that the carer’s death could not have been predicted, but might have been avoided had the firm taken proper account of her inexperience.
A Glasgow City Council spokesman said: “Our records show that the pursuers in this case abandoned their action against the council on March 16 this year.”