The disappearances of Bennylyn Burke and her two-year-old daughter Jellica are now being treated as a murder investigation, Police Scotland said.
A 50-year-old man from Dundee has been arrested and charged in connection with the deaths of Ms Burke and her daughter.
Officers are still searching for their bodies.
The 25-year-old mother and her two children were reported missing from their south Gloucestershire home on Monday March 1, having been last seen on February 17.
A further update has been provided by @TaysidePolice this evening in relation to a missing persons enquiry, involving a family from South Gloucestershire.— Avon and Somerset Police (@ASPolice) March 6, 2021
Officers from Avon and Somerset Police are offering support to Bennylyn's next of kin.
Full details below: https://t.co/v2JDy15jYq
Ms Burke’s other child has been found and is being supported, according to police.
Avon and Somerset Police released an appeal for information on Wednesday.
On Friday, a man was arrested following a raid at a property on Troon Avenue in Dundee at approximately 3.20pm.
Detective Superintendent Graeme Mackie, of Police Scotland’s Major Investigation Team, said: “We are now treating the disappearances as a murder investigation and I would urge anyone with any information to please come forward and speak to us.
“This is a complex inquiry involving both local officers in Dundee and specialist resources from across Police Scotland.
“Bennylyn’s next of kin have been updated on the arrest and are being supported by colleagues from Avon and Somerset Police.
“Our thoughts are with her family and friends at this very difficult time.
“There will be an increased police presence in the area for a considerable period of time and I would like to thank the local community for their cooperation.
“Anyone with concerns or information can speak to a local officer or call Police Scotland via 101, quoting incident 1434 of March 5 2021.”