Sarah is a help from 1920. I am thinking from what I am seeing that Agnes Robertson McMillan had died by 1913 when husband, widower, married a Sarah Carmichael.
However, you mentioned much earlier on that Alexander was born 1880? The Alexander McKellar, shipmaster,widower, who married in 1913 was aged 48. Parents Alexander McKellar a seaman and Christine McArthur, both deceased.
From the marriage you have for Alexander to Agnes Robertson McMillan, what were his parents' names?