"The paterfamilias has a mother."
Translation:Paterfamilias matrem habet.
It's not forgetting, it's on purpose. The same way you would use "samourai" or "maharadja", it's a cultural thing, a paterfamilias is a concept that doesn't exist in our cultures. It's a law term.
It's not only "the head of the family" or whatever, being "paterfamilias" is a legal status in the Roman law, conferring to the person the right to kill any member of his family, or to sell them as slaves... Do you really think it's the same thing that a modern father or head of the family?
Fun fact: This also could be written in Latin as "Mater patrifamilias est." This is using a construct called a possessive dative. This sentence literally means "There is a mother to the paterfamilias". Note that "mother" is in the nominative and "paterfamilias" is in the dative. Translating the idiom (which is quite common in Latin) into English gives "The paterfamilias has a mother".