Marriage date from family Bible.

1920 U.S. Census, Baltimore City, Maryland
6th Ward, Enumeration District 71, page 11A
107 Streeper
DeKeen, Sylvester, head, owned, mortgaged, male, white, 44, married, can read and write, born in Maryland, both parents born in Maryland, speaks English, laborer in smelting company, wage worker
DeKeen, Marie, wife, female, white 44, married, can read and write, born in Maryland, both parents born in Maryland, speaks English, no occupation
DeKeen, Chas, son male, white, 16, single, in school, can read and write, born in Maryland, both parents born in Maryland, speaks English, no occupation

On 17 October 1923, The Baltimore Sun published real estate transactions including Robert J. Simpson to Mary B. Dekeen, etc., n.s. Hudson, 1 lots g.r. $42 … 1
[The Baltimore Sun, 17 October 1923, page 19.]

1936 Baltimore City Directory, page 329:
DeKeen Sylvester W (Mary B) plmbg supplies 3000 Hudson h do

1937 Baltimore City Directory, page 308:
DeKeen Sylvester W (Mary) plmbr supplies 3000 Hudson h do

1940 U.S. Census, Baltimore City, Maryland
1st Ward, Enumeration District 4-35, Sheet 11
3000 Hudson Street, household 303, owned, value $3,500, not on a farm
DeKeen, Sylvester W., head, male, white, 58, married, not in school, highest level of education 5th grade, born in Maryland, living in same house in 1935, at work week of March 24–30, worked 54 hours that week, hardware store owner, works on own account, 52 weeks employed in 1939, has additional income
DeKeen, Mary B., wife, female, white, 61, married, not in school, highest level of education 5th grade, born in Bohemia, naturalized, living in same house in 1935, not at work, not seeking work, engaged in housework, has no additional income

On 6 May 1943 at 11 p.m., Mary Barbara DeKeen died at Harford Convalescent Home, 4700 Harford Road, Baltimore, Maryland. She had stayed there for 3-1/2 months and had lived in Baltimore for 52 years. Her residence was 3000 Hudson Street in Baltimore. She was female, white, married to Sylvester DeKeen. Born on 11 March 1877 in Czechoslovakia, aged 66 years 1 month 25 years, occupation housewife. Father was Albert Schimunek, born in Czechoslovakia; mother’s maiden name was Jedlicka, born in Czechoslovakia. Informant was Sylvester DeKeen of 3000 Hudson Street. Buried at First Evangelical Cemetery on 10 May 1943. Funeral director was Charles E. Schimunek of 2601 E. Madison Street. Cause of death was cerebral hemorrhage due to arteriosclerosis and arthritis deformans[?]. Other conditions [first illegible], ventral hernia. [Baltimore City, Maryland, Certificate of Death G04156.]

Her headstone states:
Mary B. DeKeen
1877-1943