Who was Isabel Mellinger and what happened to her?

Question

Isabel appears as the 13 year ol daughter of Bill and Nell Mellinger in the 1930 census.  She does not appear in the 1920 or 1940 census.  Family history says Bill and Nell did not not have children.  The house at 1153 17th Avenue in Columbus was left to her great nephew Charles Thomas Wilkins at the time of Nell's death in 1966.

Was Isabel a foster child?  Was she a relative?  What happened to her after 1930?

Comments

There was another Isabelle Mellinger in Canfield, OH in 1996     https://familysearch.org/ark:/61903/1:1:QJ21-L66C

The 1930 census has Isabel birth estimated as 1917 and her mother as Ella I. Are you sure that Family History is correct on Bill and Nell not being Isabels parents and that the 1930 Census is incorrect on that subject?

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There are a few things that do lead me to believe that family history passed down was correct:

1)  Isabel does not appear with Bil and Nell in the 1920 census

2)  There is no mention of a child dependant on Bill's WWI draft card/

3)  She does not appear with Bill and Nell in the 1940 census -- cannot locate a marriage or death record.

4)  She is not at Union Cemetery with Bill and Nell

5)  Assets and heirlooms were left to extended family members when Nell died in 1966.  The house was left to my uncle.  I now have her chocolate set -- that was left to my grandmother.

I am not familar wth the Mellinger family so perhaps a niece --- not sure.  Or perhaps a foster child.  I do need to locate Bill and Nell's obituaries to see if there is any mention of her.

Nell was very close to my grandmother -- as her mother died young from complications from surgery.  The family was very close and small in that regard.