Wow, what a hard family to find, but I think I found the right people. First I looked for a Nancy Long in 1930 census and found a Nancy Long age 49, mother-in-law and Elmer L. Long, 56, father-in-law living in the home of Frank and Bertha Hammond. They were in Magnolia, Mingo Co. WV Nancy is shown as first married age 16, Elmer as first married age 22.
I then looked for them in earlier records.
1920 they are in Red Jacket, Mingo LONG Elbert Lee age 44 Nancy age 41 in same house WILLIAMS George, age 17, step son Cecil age 14, step son
next door HAMMOND Frank and Bertha and living with them is John Browning, grandfather (to Bertha I guess)
I can give you full entries if you like.
1910Rock, Mercer WV LONG Elbert age 34 Nancy age 28 Lem (from index) but looks like Livy to me, daughter, age 12 Bertha daughter age 9 George son age 7 Cecil son age 4
notice in this census George and Cecil are listed as sons, but in 1920 as step sons. It looks like Nancy may have been married before to a Williams.
I tried to find Nancy in 1900 . I found Elmer who looks like he was married to someone else at the time. Elmer age 30 Eliza age 24 Arthur age 4/12
I believe I found Nancy in 1900. She is living almost next door to her parents John and Sarah. She is married to W.R. Williams age 28, born KY May 1872 Nancy age 19,born WV Oct. 1880 ,married 4 years, 1 child born, 1 living Leavie, daughter age 1, born June 1898
I know the year of her birth does not agree with your date of 1878 but the death cert. may be wrong. Whoever gave the information may not have known her actual date of birth. 1880 makes more sense as I looked for her in 1880 census and found her parents but she was not there. If she had been born 1878 she would have been with the family.
I suppose Leavie born 1898 is likely Livy from 1910 census, so she and other children in 1910 were listed as Long when they were actually Williams.