The cognate sets for "mother", "father", "brother", "sister", and "daughter" are popular in introductions to the concept of Proto-Indo-European because this "core vocabulary" is so consistently preserved in the daughter languages. But just because a lexeme persists doesn't mean its original meaning does.
I learned this lesson anew when writing
this entry in Irish. Based on those lists of kinship terms, I reached for
mo bhráthair when I needed to translate "my brother". But in Modern Irish,
bráthair is used solely in to refer to clergy and the usual term for "male sibling" is
deartháir.
Irish is hardly unique among European languages in this respect. The same is true in Italian, for instance, where the direct descendants of Latin
frater and
soror (respectively
frate and
suora) persist only in religious titles and the terms siblings are presently known by are diminutive in origin (e.g.
fratello < Latin
fratellus "little brother").
In Ibero-Romance, the words for "brother" are derived from
germanus, originally an adjective meaning "of the same parents". This distinction may have been necessary not so much because remarriage was rife but because
frater also had the meaning "father's brother's son" (
frater patruelis), i.e. a kind of first cousin.
So what
is the origin of Irish
deartháir (and its female counterpart,
deirfiúr)? When I learned that genitive of
deartháir is
dearthár, it tripped my suspicions that there was some sort of relationship to
bráthair after all, since that's a form of declension that's--as far as I know--limited to the PIE kinship terms.
A look at my (Scots) Gaelic dictionary turned up
dearbh-bhráthair as a term for "blood brother" (as opposed to stepbrother, foster brother, etc.); the element
dearbh has the meaning "actual, true". The exact phonetic developments are unclear to me--
dearbh-bhráthair contracted to *
dearáthair and thence, by metathesis, to
deartháir?
The story is much clearer with
deirfiúr, which would represent a regular outcome of *
dearbh-shiúr: Irish
sh is pronounced [h] which would merge with and devoice preceding [v] [spelled
bh], yielding [f]. The only snag seems to be that I can't find evidence of this pronunciation in any dialect!
In Munster, you have metathesis of unstressed
dear- to
drea-, but this is a regular process in the dialect (cf.
turas "journey" >
trus). The broad nature of the /f/ is probably explained by the fact that most Munster dialects seem not to distinguish broad and slender /h/, so the change would've been /dʲarv/ + /sʲu:r/ > */dʲarv'hu:r/ (rather than */dʲarv'hʲu:r/) > /drʲa'fu:r/.
But I'm at a loss to explain what happens in Cois Fhairrge, where the pronunciation is /'drʲaur/. If I had to phoneticise this, I'd use the respelling *
dreabhar, but Ó Siadhail inexplicably prefers
driofúr, which is neither Caighdeán Oifigiúil nor reflects the dialect pronunciation.