Never assume that the heirs of one person are necessarily all heirs of their spouse they had at death. It is easily possible that there were multiple marriages by either the husband or the wife. This could result in them having different heirs.
But a missing heir when the surviving spouse dies could simply mean that that heir died before the surviving spouse did and that heir left no descendants of their own.
Bottom line: compare heirs of the husband and wife if you can to find clues about potential multiple marriages.
Learn more about US probate records in my class.