FamilySearch database of the 1880 Census, Twp. 104, Minnehaha, DT:
OLSON, Nels, 38, NOR NOR NOR; wf.
Ragnilda, 27, NOR NOR NOR; ch. b. Dakota:
Martha, 6;
Ole, 4; &
Carl, 2.

Nearby was another OLSON listed as Torbjom, 57. Same twp. has
BARSNESS, Knudt, 57, NOR NOR NOR; wf.
Berta, 56, NOR NOR NOR; son
Torsten, 29, NOR; ch. b. Minnesota,
Johanna, 18, &
Lars, 17.

Grave Records of Minnehaha Co., SD {by Sioux Valley Gen. Soc.},
Cem. 11, Dell Rapids (town & township), Sec. 4, BOYUM
Karl O. (1878 - 1953),
Julia (1883 - 1964),
Walter E. (1910 - 1956)

Cem. 43, St. Paul's Lutheran, Garretson, Edison Twp., Sec. 17.
BOYUM, Theresa M., d. 1898
Bereth, d. 1912
Nels O, d. 1924.

St. Jacobs, Colton, Buffalo Twp., Sec. 15: OLSON, Rognald, d. 8 Aug. 1904.

There are many BRAA burials in North Highland Lutheran Cem., Sherman, Highland Twp. Sec. 17. The eldest for whom dates are listed is Stephen BRAA, d. 21 Je. 1900, aged 84.

There's Leonard BRAA, d. 1906, bu. St. Paul's, Garretson, only BRAA listed there.

There should be death certificates for those who died mid-1905+. If you order from the state, you need to show relationship. The county may not require that.

SD Births (online--see site for limitation) has

Nels Olson BOYUM & Theresa BRAA, sons
Edward Theodore BOYUM, b. 25 S 1890, &
Melvin Arthur, 1 Apr. 1893.

Karl BOYUM & Julia THOMPSON had Norvald, b. 24 Je. 1905. Norvald was still of Minnehaha Co. in 1930, with Margt., b. ca. 1905; Marjorie, b. ca. 1927, & Norvald J., b. ca. 1928.

Social Security Death Index has, last res. Sioux Falls, SD: Robert BOYUM (13 Jan. 1936 - 26 Oct. 1993) SSN issued SD.

This is on FamilySearch, but I haven't verified it. Arne Eirikson BOYUM (1794, Boyum farm, Mundal parish, Balestrand, Sogn Og Fjordane - 1834, Balestrand, s/o Eirik Arneson BOYUM & Brita Ellensdatter TUFTE) m. ca. 1834, Balestrand, Brita Jonsdtr. SKELE, b. ca. 1805. They had
Eirik, Nils, & Ellend.

Good Luck!