The word for sister in Irish is deirfiúr /dʲɾʲəˈfˠuːɾˠ/, and it has always puzzled me why this word is so different from the words for sister in the other Gaelic languages: piuthar /pju.ər/ in Scottish Gaelic and shuyr /ʃuːr/ in Manx.
Yesterday I discovered that deirfiúr is in fact a combination of deirbh /dʲɾʲəv/ (true) and siúr /ʃuːɾˠ/ (sister). The word siúr originally meant sister in Old Irish, but came to mean kinswoman. To distinguish sisters from other female relations, deirb (true) was added to it, so the Old Irish word for sister was derbṡiur, which eventually became the Modern Irish deirfiúr – the s at the beginning of siur became f after mo (my), do (your) and a (his), and this mutation became fixed.
In Scottish Gaelic the word for sister came from Old Irish as fiur, which became piur and eventually piuthar.
The Old Irish word siur (sister) comes from the Proto-Celtic *swesūr, from the Proto-Indo-European *swésōr, which is the root for the word for sister in many European languages.
The Irish word for brother, deartháir /dʲɾʲəˈhaːɾʲ/, has a similar history: it is a combination of deirbh (true) and bráthair (brother) and used to be written dearbh-bhráthair or dearbhráthair. It comes from the Old Irish derbráthair, from the Proto-Celtic *brātīr, from the Proto-Indo-European *bʰréh₂tēr. In Modern Irish bráthair means brother as in a male member of a religious community or monk. In Old Irish it meant brother, kinsman or cousin.
Sources: Blas na Gàidhlig: The Practical Guide to Scottish Gaelic Pronunciation, by Michael Bauer