Police were called to the home of Katherine Helene Panin, 81, following a report a woman had been injured after falling down stairs on October 12.

She later died in hospital.

ACT Policing said in a statement said officers launched an investigation and searched two Canberra properties and three vehicles in June 2016.

Police charged Ms Panin's daughter-in-law, Melissa Beowulf, 60, and her son Bjorn Toren Beowulf, 29, about 9.30am on Friday.

The pair were formally charged with murder when they faced the ACT Magistrates Court on Friday afternoon.

Defence lawyer Peter Woodhouse entered not guilty pleas on their behalf. They did not apply for bail.

The mother and son will return to court in December.

A second man, aged 31, was arrested in NSW and was being extradited to the ACT. He is expected to face court, charged with murder, on Saturday.

In the police statement, Detective Superintendent Ben Cartwright praised the dedication and commitment police showed during the "complex" 23-month inquiry.

"The complex nature of homicide investigations, such as these, require expertise, commitment and immense sensitivity.

"The community trusts us to bring these offenders before court and we need to provide some closure to the family and friends of Katherine."

Information to Crime Stoppers on 1800 333 000 or at act.crimestoppers.com.au.