I'm thinking Angus McMillan was the son in law, married to Ann McMillan?
Peggy came from Plantagent Twp, strange she died back there. Maybe they bought a little place back there after they sold the farm to John Kennedy?
I stumbled across a listing of McMillans in area, will post here incase anyone else is searching.
http://www3.sympatico.ca/comflex/mcmillan/Belden1879.htm