That likely sorts out John Cochrane

His DC and last Will is on SP:
15 March 1889 -
Glasgow, afterwards Inspector of Public Buildings, Master of Works Office, Glasgow, thereafter residing in Helensburgh, d. 29/01/1889 at Helensburgh, testateJohn's death in 1889 in Row/Rhu gives mother's maiden name as Arnott (as we know, info is only as good as the informant gives at the time).
Likely sisters' deaths (showing as Cochran and mother's maiden name of Arroll) are Ann who died in Glasgow in 1883 and Isabella who died in 1888 in Row/Rhu.
Still no clues for Margaret Whitehead I think

Monica