Judah and Simeon These are mentioned together, because the cities of Simeon lay within Judah's portion. Families That is, of the family, the plural number being put for the singular which is not unusual.
Related Commentaries of Joshua 21:9
Joshua 21:9-10
9 And they gave out of the tribe of the children of Judah, and out of the tribe of the children of Simeon, these cities which are here mentioneda by name,
10 Which the children of Aaron, being of the families of the Kohathites, who were of the children of Levi, had: for theirs was the first lot.