There is a difference between being someone's heir and being their descendant. A descendant "descends" from the person--is their child, grandchild, great-grandchild, etc.
An heir is someone who (usually according to state statute) is entitled to a share in a deceased person's estate. If the deceased person had living children, they are usually heirs. The children of a deceased child would also be heirs.If the deceased had no children, grandchildren, great-grandchildren, etc. (and left no surviving spouse) then their heirs could be their siblings, or their first cousins, depending upon the family structure.
And of course, the definition of heir is dependent on statute--so check that out for the time period of interest as well.