I've only been able to get hold of Murder on Page Three so far, The Water Widow being out on loan (only 1 copy of each in the system). Her series characters are Oslo-based Detective Sergeant Rudolf Nilsen and his brother Detective Constable Karsten Nilsen.
Here are the front and back flaps (which are clear if you click on them) revealing a bit about the plot and the author.
Murder on Page Three was published in Norwegian in 1982 and in English in 1984 by Quartet Qrime. The translator is J Basil Cowlishaw.
The Water Widow was published in English in 1986 and I've also found reference to a short story collection called The Dead Don’t Steal and the title story was made into a Tales of the Unexpected episode in 1988.
Here're the first few paragraphs from Murder on Page Three:
Lucky was getting restless. He wanted to go out. Karin pretended not to notice; she was trying to think. The sheet of paper in her typewriter was as pristinely white as when she had inserted it over an hour ago.Here's the blurb from The Water Widow:
"Make sure there's a murder in the first chapter," her publisher had said. "The best would be on page three."
She had objected that she wasn't a tailor, running up suits made to measure, at which he had laughingly countered with: "Maybe not, but you are Norway's Agatha Christie, and you have a reputation to keep up".
The case begins when a fifty-five-year-old shop assistant with a raging toothache visits the dentist. He's shown into the surgery by a tall woman dressed in widow's weeds, and is never seen alive again."