Private User's father grew up as the son of Jacob and Mary Siegel.
The 1920 Census shows Jacob and Mary Siegel in New York City with 4 children, all girls, the oldest, Rose, age 14, the youngest under a year.
The 1930 Census shows Jacob and Mary Siegel in New York City with 4 children - the younger 3 from the previous Census, plus Herbert, age 4.
Rose would have been about 20 when Herbert was born.
There certainly have been instances where a couple chooses to legally or unofficially adopt and raise a grandchild as their own child.
Private User - is it currently your belief that Herbert was the biological child of Rose and David Walter Bloom?