Communication was more difficult in the 18th and 19th century than it is today. It is possible that your ancestor had brothers and sisters who moved away from home and were never heard from again. Or it is possible that they did keep in some sort of contact. It varies from one family to another.
I've seen Civil War pension files from the US where the person had no idea where their siblings were living and I've seen others where they knew where everyone was. Some families kept in better contact than others.
All of which makes some people more difficult to find--but don't be surprised if great-great-grandma doesn't know what happened to most of her siblings. It really is possible.