The immediate family members in Scottish Gaelic are màthair, athair, bràthair, all of which are clearly related to other familiar European languages, and piuthar, “sister”, which looks odd. Irish is yet odder at first glance, with deartháir meaning “brother” and deirfiúr meaning “sister”.
I’ve been reading David Stifter’s Sengoidelc, a readable and reassuring text about Old Irish, the written Irish of the 8th and 9th centuries, which contains at least part of the explanation. It turns out that in Old Irish there were two letters s. One of them lenited by turning into an h, a bit like s in Gaelic becoming sh pronounced /h/, but the other one turned into an f, and the main word that began with that sort of an s was siur, meaning “sister”.
What seems to have happened in Scotland is that the nominative case form was back-derived from the lenited form phiur and assumed to be piur. Conversely in Ireland the nominative form won out, and they say siúr, but mainly, I think, for non-biological sisters, like nurses and nuns. A further difference here: Scotland retains the disyllabic form, whereas in Ireland it’s been simplified to a long vowel.
But why in Ireland do they say deartháir and deirfiúr for your biological siblings? Enter eDIL, the Electronic Dictionary of the Irish Language, which has entries for derbráthair and derbsiur, “true brother” and “true sister” respectively.