I can find nothing on death and burial of Joseph, census records indicate he died between 1920 and 1930. Joseph and Elizabeth were married on October 19, 1891. Marriage license docket states he was a widow at the time of marriage, it is possible children Joseph and Agnes's mother was unknown first wife. 

1900 Census:

Joseph Miller (Head): age 42, born Aug 1857, Hungary. Residence: North Braddock, Allegheny, PA. Immigration Year: 1890. Marriage year: 1891. Occupation: Mill Laborer

Lizzie Miller (Wife): age 37, born Dec 1862, Pennsylvania. 

Children living in the household: Joseph (18-1881), Agnes (15-1884), Adolph (8- 1892), Bertha (5-1894), Clarence (at some point he became Charles) (4-1895) and Robert 3 months.

1910 Census:

Joe Miller (Head): age 53, birth 1857, Germany. Residence: Philadelphia Ward 19, Philadelphia, PA. Immigration Year: 1890. Occupation: Conductor, Street Railway.

Elizabeth Miller (Wife): age 46, born 1864, PA.

Children living in the household: Adolf (17), Bertha M. (15), Clarence (13), Robert F. (10).

1920 Census:

Elizabeth Miller (Head): age 56, birth 1864, PA. Residence: Philadelphia Ward 43, Philadelphia, Allegheny, PA.

Joseph K. (R. resembles a K) Miller (Husband): age 63, birth 1857, Austria. Occupation: Conductor, Industry Tailor

Children living in the household: Bertha (25), Clarence (23), Robert (19), Marie Miller (5) shown as daughter (most likely Robert's daughter).

1930 Census:

Robert Miller (Head): age 30, birth 1900, PA. Residence: Philadelphia Ward 42, Philadelphia, Allegheny, PA

Florence Miller (Wife): age 26, birth 1904, PA.

Children living in the household: Marie Miller (16), Francis (8)

Elizabeth Miller (MIL): 68, birth 1862. Widowed

Bertha Miller (Sister): 35, birth 1895, PA. Single. Occupation: Winder, Sweater Mill