This Condition located on the Family category tab in Search Builder requires that you know the Family ID # for the family. It is the number at the end of the URL when you are viewing the Family Page for a family.
Enter the ID # and the results will be everyone in that family.
You can select Greater Than and enter a number to find families whose family ID # is greater than the number indicated if you are looking for families whose records were created after another specific family.
The easiest way to find everyone in the same family is to click the Family Members link on the people record of anyone in the family.
If you want to include the extended family, click the Related Families link.
Each of these links will take you to Search Builder with the search built for you. The arrow will expand the selection, and the link will build the search.