I, also, was fascinated as to how this could have come about and discovered a bit more at this link on thepeerage.com, a well-respected and usually reliable source,
https://www.thepeerage.com/p41343.htm#c413424.1
Interestingly, the mother of the bride wasn't directly a Forbes of Craigievar but came from a cadet branch descending from a younger son born in the mid eighteenth-century.