What’s the genitive of pater familiās? Is it pater familiātis? patris familiātis?
Actually, it’s patris familiās.
The second word, familiās is an archaic partitive genitive, so it’s not declined.
- pater/patris/patrī/patrem/patre/patrēs,etc. familiās.
However, if we’re discussing the father of many families…
- pater/patris,etc. familiārum
The Essential AG: 43b