In the U.S. in 1917 and 1918, men were required to register for possibly being drafted into military service for World War I. The first registration was on 5 Jun 1917 for men that were born between 6 Jun 1886 and 5 Jun 1896 and among the registrants was Bror. His draft card is online at https://familysearch.org/pal:/MM9.3.1/TH-1971-26462-17885-10?cc=1968530&wc=MMTJ-TZ2:n1425822939
In the 1920 census, Bror had married Hulda SANDSTROM and they lived in Saybrook Township, Ashtabula County, Ohio: head, Bror Albin CARLSON, 25, immigrated 1915, *PA, born in Sweden, both parents born in Sweden, occupation car repairer-car shop (probably for a railroad) wife, Hulda H., 32, immigrated 1910, born Sweden, both parents born Sweden stepson, Thure A. SANDSTROM, immigrated 1912?, born Sweden, both parents born Sweden stepson, Carl G. SANDSTROM, 8, born Ohio, both parents born Sweden
*PA is Papers Applied; first papers filed for citizenship.
Per the information in the 1920 census, Hulda married first in Sweden, had Thure ca 1909, then came to the U.S. and had Carl ca 1911. The given name of Mr. Sandstrom is unknown but may be available from the sons' death certificates, their obituaries or their Social Security application forms.
Bror, Hulda and Lennart are still in Saybrook Township in the 1930 and 1940 censuses.
In the 1930 census, Thure had married Florence ca 1929 and they lived in East Cleveland. In the 1940 census they lived in Cleveland Heights and had Carol J. age 8 and Kenneth age 6.
In 1930, Carl had left the home but lived in Cleveland and by 1940 he had married Violet and they had a daughter Phyllis age 4 and lived in East Cleveland.
This is Carl Sandstrom's obituary, which has some good information, except for 2 possible typographical errors which I marked with an asterisk and enclosed in parentheses: Date: Dec 22 1974 Source: Plain Dealer; Cleveland Necrology File, Reel #152. >Sandstrom, Carl G., beloved husband of Violer F. (nee Snea *SHEA), father of Mrs Phyllis Echt (*HECHT) and Frazier, grandfather of Craig and Lance Echt (*HECHT), brother of Lennart Carlson of California, Thure Sandstrom of Florida and the late Ernest Carlson, Dec. 15, 1974. Services were held Wednesday, Dec. 18. Arrangements by Maher-Melbourne Inc.
Ernest Carlson was born 8 Jun 1920 and died 21 Jun 1925. His death certificate is online at https://familysearch.org/.
Hulda and Bror both died in Ashtabula, she on 17 Mar 1969 and he on 1 Jul 1971. I can't find any obituaries online for them but you could email the Ashtabula library and ask, if they have their obits can they scan copies to you. Check http://www.acdl.info/http://www.acdl.info/ for more information.
Thure died28 Jan 1982 in Broward County, Florida,possibly in Pompano Beach.