Hi John
Well done on finding John's DC. Re his family, can't see a marriage for his parents on IGI. There is however one sibling showing for John, an actual extract from the OPRs:
DONALD FERGUSON Christening: 14 MAY 1804 Clyne, Sutherland, Scotland
Given the probable ages of the parents, they may well have died by the 1841 Census. I did try looking for Hugh in that Census a few days ago and no trace of him. One possibility for Margaret, I'll go back and try and find it!
Regards.
Monica