Concerning the Woergaard estate: I should have been more precise. The dairy was on a farm (annex) that was part of the Woergaard estate. So Anna Britta was not staying in the main building.
Concerning her age: There are several Swedish workers/servants at the farm. Three are listed as 30 years old - a little suspicious. Most likely the census official asked the owner, and for people not present he just gave his estimate of the age (I've seen this before in my own family for farm workers).
Concerning the sister: There is an option that she is there but with a slightly wrong name.
Look at this list:
Johanne Britta Carlsdatter 30 Ugift Tjenestetyende Sverrig
Anna Britta Larsdatter 26 Ugift Tjenestetyende Sverrig
Auguste Larsdatter 23 Ugift Tjenestetyende Sverrig
Inge Kaise Bojsen 18 Ugift Tjenestetyende Sverrig
Anna Britta Andersdatter 30 Ugift Tjenestetyende Sverrig
Johanne Stine Andersdatter 30 Ugift Tjenestetyende Sverrig
The names could be partly incorrect (as the ages).