Hi Robert ! Welcome to the Kintyre Forum !
I’d previously researched your question but came up with nothing; but with so much info, I found it illogical that there was no mention of at least one of them.
I surfed the family trees in Ancestry and came up with one very similar family..., too similar not to be the same. But either you or the member has made a mistake with the McKinlay name. He has her down as Peggy and not Margaret. Except for your Catherine, you probably have more info than him. On the other hand, he looks as if he could also be a gg-grandson. (I could PM him if you want in order for you to share info)
John McAlister:
Birth abt 1796 in Argyll, Scotland
Death
Peggy McKINLAY
Birth abt 1791 in Argyll, Scotland
Death
Children:
Archibald McALISTER
John McALISTER 1816 –
Catherine McALISTER 1826 – 1902
Catherine McALISTER
Birth 22 Jan 1826 in Kilkenzie Parish, Argyll
Death 19 Dec 1902
Husband John McMurchy (John’s father... John 1791-1867: his mother Mary McGleachy 1797-1889 / his grandfather John married to Janet Carmichael)
Birth 22 Aug 1820 in Killean Parish, Argyll
Death 3 May 1907
Catherine’s children: (married 24th November 1846)
Donald McMURCHY 1847 –
Edward McMURCHY 1851 –
Archibald MCMURCHY 1855 – 1887
William McMURCHY 1857 –
Jas McMURCHY 1859 –
Ronald McMURCHY 1860 – 1887
All mentioned in the 41/51/61 and 1901 census. (two elderly brothers dying in the same year !?)
This does not answer your question, but I hope "Peggy" might help you.