On Paul's WWII Draft Registration card, he lists William Mattson of Berkeley as his next of kin. That's his brother.
1940 census, Berkeley, Alameda, California
William Mattson 52 Lilly Mattson 48 Leo W Mattson 28 Minnesota Henry Lampi 30 Irene Lampi 27 Billy Lampi 10/12
California, Death Index, 1940-1997 about William Mattson Name: William Mattson Social Security #: 547071070 Gender: Male Birth Date: 22 Mar 1887 Birth Place: Other Country Death Date: 6 Apr 1954 Death Place: Alameda Mother's Maiden Name: Swanstrom Father's Surname: Gunsbeak
California, Death Index, 1940-1997 about Leo W Mattson Name: Leo W Mattson Social Security #: 573144181 Gender: Male Birth Date: 7 Nov 1911 Birth Place: Minnesota Death Date: 10 Nov 1993 Death Place: Fresno