Unlike māter ‘mother’, pəter does not denote the physical parent, as is evidenced, for instance, by the ancient juxtaposition preserved in Latin Iupiter.
Benveniste establishes that the Indo-European term for ‘father’ is fundamentally a religious and social designation rather than a biological one, with the compound Iupiter as its most ancient divine attestation.
, Indo European Language and Society, 1973thesis