Hello Michelle, J. A. Isbister was my g.g. grandfather. I believe Thomas and John were brothers but can't prove it as yet. I don't know if Magnus was related. According to Charles Denney (the metis archivist) my g. grandfather George Barnston Isbister and A.K. Isbister were 1st. cousins. From this I would surmise that Thomas and John were bros.
Charles Denney (now deceased) has his collection at the Bow Valley library. I have yet to go there but am planning a trip soon.
It appears that you and I have info. that is selfsame.