Depending upon the time period and the location, it can be difficult to know for certain all the children or siblings of an ancestor. In places where there was no civil or ecclesiastical recording of birth records, knowing “for certain” you’ve got all the siblings can be a challenge. Probate records often don’t mention pre-deceased children if they left no descendants of their own. Some children may have life spans that don’t include a census year. Some families may not mention children who died young–not because of shame, but because of emotional upset.
Always look at birth dates of children and see if there’s a gap–there might have been children that simply migrated away and are accounted for, some might have died young, there might have been miscarriages, or there may have been no children at all.
If there’s a ten year gap and the last child was born when the mother was fifty, it’s possibly not her child but her grandchild.