"my sister" exceptional?

[muckefuck]

I mentioned dipping into Sengoidelc last night, but I didn't mention the new insight I gleaned from it. You may vaguely recall my post in January about the Irish word for "sister" (officially deirfiúr, dialectally driofúr or dreabhar). At the time, I thought that the f was due to devoicing of the final /v/ of the prefix, dearbh-, but Stifter's book suggests another possibility: Apparently, there were two possible lenitions of Old Irish initial /s/: /h/ and /f/. The second variant occurs only before PIE *sw-, as in--for instance--PIE *swesor "sister", e.g. OIr mo fiur "my sister". The Irish soon ironed out the apparently irregularity in favour of /h/, but it's just possible that the /f/ in deirfiúr is a relic from a theoretical Old Irish *derbfiur.