If Robbie Wilson is right and your mother-in-law was born in Muirkirk, Ayr in 1918, then there is a Hugh Strathearn b Muirkirk in 1904 (d 1985 Kilmarnock) and a James Strathearn b 1912 Muirkirk (d 1986 Old Cumnock which isn't far from Muirkirk). Muirkirk itself is not a large place (have you been there?) so that the coincidence of all 3 of these family members being born there within a similar timeframe suggests they may have been siblings! Of course you would have to order up the certificates to prove who this Hugh and this James might be, but then you would have your answer. As another person here has suggested, Hugh Strathearn may also have been a doctor. Do you know the possibility of that? Kilmaurs is virtually next door to Kilmarnock where he may have died. Some correct dates I believe have been lost in transcription!