It has emerged the couple had split up and Ms Rennie reportedly moved out of the family home several weeks ago.
Superintendent Gary Flannigan, of Fife Constabulary, said the incident looked to be "entirely self-contained" as police dealt with one death and one murder.
He said officers were not aware of any previous incidents involving the couple, who are thought to have had been together for about six years.
"We are dealing with a tragedy for two families," he said.
Mr Flannigan, of the Serious Crime Division, added: "Events began with a call to a house in Rosyth, and that led us to the house in Dalgety Bay. We are fairly confident we know what we are dealing with. There are two children at the centre of this tragedy."