It looks like the Susan Davidson you mentioned is linked to Mabel through her mother Eleanor's second marriage to Alexander Stuart Harrington and so Susan Davidson is his sister.
The Caledonian Mercury (Edinburgh, Scotland), Friday, October 20, 1865
Marriage
DAVIDSON - HARRINGTON - At Torrance Lodge, on the 17th inst, by the Very Rev. Dean Henderson.
JAMES DAVIDSON Esq, Major 1st Inniskilling Dragoons, to SUSAN, only daughter of the late Robert HARRINGTON, Esq of Terrance Lodge, co Lanark, and granddaughter of the late Sir John Harrington, Bart, of Ridlington, co Rutland.
There is a tantalising snippet here = Burke's Peerage Limited, 1914
http://www.rootschat.com/links/0lfo/I can see it mentioning Alexander Stuart (Harrington) marrying Elinor widow of Thomas Walker of Eastwood Hall... then underneath mentions Susan m 16th Oct 1865 James Davidson