Hi Carolbea,
The John Boyd & Isabella McGregor 1869 marriage cert certainly does record Isabella’s parents as John McGregor & Christina McPhee.
I Suspect that the link is through the McPhee line: Isabella’s mother, Christina McPhee and Donald McLachlan’s wife, Marjory / Misey McPhee being sisters but have not yet found birth records for either.
From
https://familysearch.org/search/Children of Donald McLachlan &
Marjory McPhee:
Hugh McLachlan 29 June 1836 Morvern
Donald McLachlan 28 May 1838 Morvern
Alexr McLachlan 25 Dec 1839 Lismore & Appin
here is the family in 1841: 525/ Lismore & Appin, Glensanda
Dond. McLachlan / 45 / Grieve
Misie (Marjory) McLachlan / 35
Children: Hugh, Donald, Alexander
Mary McGregor / 18 F. servant
*All born Argyll
Isabella in 1841: Camusallah, Parish 528, Morvern
Dugald McPhee / 85
Hugh McPhee / 75
Mary McPhee / 45
John McGregor / 40 Ag Lab
Christy McGregor / 40 (m.s McPhee)
Catherine McGregor / 14
Ann McGregor /12
Isabella McGregor / 8
Jessy McGregor / 5
Catherine McPhee / 35 F. Servant
*All born Argyll
1851 census, Parish 525, Lismore & Appin, address: Glensanda
Donald McLachlan / 54 / labourer & cottar
Miller (Marjory) McLachlan / 47
Hugh McLachlan /14
Donald McLachlan / 12
Alexr McLachlan / 10
Dugald McLachlan /8 / Lismore & Appin
Bell McGregor / 16 / niece / house servant
*all born in Morvern apart from Dugald