1920, living at 21 Grant St, Newark Ward 4 > District 136 > 12, listed as Mary Dupa, mother-in-law to Paul Cumming (HOH), so mother of Paul's wife Martha. Listed right after her is George Decker, listed as brother-in-law to Paul Cumming, so Martha's brother. Mary Dupa is age 62 (abt 1858) & widowed. This one says she was born in New Jersey, both parents Ireland.

1930, living at 114 ?? Place, Newark (Districts 647-680) > District 653 > 24. (sorry, not familiar with Newark street names and it's a poor image), listed as Marry Decker, mother, living with same family (the surname is spelled Cummins now & Martha is listed as Margarett), age 73 (abt 1957), widowed. This one says she was born in New York, both parents Ireland.

Using the birth dates & places of Mary's children (Martha/Margarett and George) as Pennsylvania, there's a family that fits living in Pennsylvania > Lackawanna > Scranton Ward 14 > District 0104 > 4. Mary Decker is HOH, age 53 (abt 1857) widowed, born in NY, both parents Ireland; son & daughter, George & Martha, of correct dates, Martha was married with last name Howard. This fits fairly closely with Martha's age at first marriage in 1930 being listed as 21. Mother of Mary Decker is living with them, her name is Martha Bensley, widowed, born Ireland.

Allowing for the usual errors (for example in 1920 George & Martha's parents' places of birth are listed as Pennsylvania, while Mary's is alternately listed as being born in New York & New Jersey in 1920 & 1930), it appears to me Bensley is Mary's maiden name. I don't know where Mary's last name of Dupa in 1920 fits in, if it, too, is not just another error.

If this is correct, and being that George & Martha were born in PA, you may want to look in PA for a marriage record for Mary Bensley to someone Decker. Sorry I didn't note her older son's name.

I did note that twins seem to run in the family. I didn't note the names but both Mary Decker & daughter Martha seem to have had twins. (I say seem to have had because on one census Martha's daughters aren't listed as the same age while they are on another.)

Anyway, I hope this helps. Another option would be to hire a researcher or get a volunteer to go to the NJ State Archives for a death certificate for Mary Decker, which should list her parents if her daughter knew their names. Since she was 73 in 1930 there's a good chance she died before 1940. The Archives has them up to 1940 and they're available to the public but only in person, not via mail. The Genealogical Society of New Jersey has a list of researchers available for hire in a brochure linked here http://www.rootsweb.ancestry.com/~njgsnj/profresearch.html or the Archives are in Trenton, Mercer, New Jersey, if you'd like to post on that message board looking for a volunteer.

Hope this helps and good luck.
Jo

In case it helps, I went back and got more info.

Also found this for 1900, and here are the family members for the other years.

1900 Scranton Ward 14, Lackawanna, Pennsylvania
H D Luther 45
Mary Luther 43
Robert D Luther 22
Wm H D Luther 20
Calvin C D Luther 18
Clarence B D Luther 14
Mattie L D Luther 11
Geo L D Luther 2
Martha Bensley 68

Data for Martha Bensley:
Birth Date: May 1852
Birthplace: Ireland
Race: White
Gender: Female
Immigration Year: 1850
Relationship to head-of-house: Mother-in-Law
Father's Birthplace: Ireland
Mother's Birthplace: Ireland
Mother: number of living children: 3
Mother: How many children: 4
Marital Status: Widowed

1910 Scranton Ward 14, Lackawanna, Pennsylvania
Mary Decker 53
William H Decker 28
Calvin G Decker 28
George A Decker 12
Martha Howard 21
Benjamin Bennet 72
Thomas Mosser 32
Martha Bensley 78

1920
Paul Cumming 31
Martha Cumming 31
Paul Cumming 3
Caolyn Cumming 2
Evelyn Cumming 1
Mary Dupa 62
George Decker 22

1930 Newark, Essex, New Jersey
Paul G Cummins 39
Margerett Cummins 40
Paul Cummins 13
Evelyn Cummins 12
Carolyn Cummins 12
Robert Cummins 10
Margarett Cummins 8
Helen Cummins 7
Marry Decker 73