I do find a link with Tom Oas, who was also listed as a boarder in 1900. There can be various reasons why the two would be listed as boarders, perhaps they didn't get along with their step-dad or some other reason.
1930 census for Menomonie, Dunn County
Gerhart Gunnufson, age 32, head
Gertie Gunnufson, age 39, wife
Ever Dahl, age 59, father in law
Anna Dahl, age 62, mother in law
Tom C. Dahl, age 26, brother in law
Ever A. Dahl, age 23, brother in law
Agnes L. Dahl, age 21, sister in lawhttps://familysearch.org/pal:/MM9.1.1/X9QC-8GG
1940 census, Menomonie, Dunn County
Tom Oas, age 56, Widow
Gordon L. Oas, age 18, son
Gertrude Gunnufson, age 49, sister, widowedhttps://familysearch.org/pal:/MM9.1.1/K7VD-BVS
Here is Gertrude's deathhttps://familysearch.org/pal:/MM9.1.1/V89K-XXL
If you check earlier censuses, it appears Gertrude and her husband had no children. Tom Oas appears in earlier censuses with his wife and children.
Are you familiar with www.findagrave.com
- you can find Anna Dahl buried here with a death date of June 17, 1952.
If you click on the name of the cemetery, below where it says burial, you can search for all others who are buried there (note, there is no guarantee all people buried there are in this list)http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GSln=da...
You might consider putting a request for an obit look up for Anna on the Dunn County message board, that should list her children. The cemetery link has some biographical information, but only lists her children by Ever Dahl.