Just to clear things up (or maybe confuse them more)...
The IMDB bio of Diane Polley (nee McMillan) states that she had one child with first husband George Deans-Buchan. Obviously, this would be John Buchan. Also, she was "Mother-in-law of Susy Buchan", which would make Susy John's wife, not his sister.
With second husband Michael Polley, Diane's profile says she had five children, three of which were Sarah, Joanna and Mark.
However, the bio for Michael Polley credits the couple with parenting only four offspring: Sarah Polley, Joanna Polley, Mark Polley and John Buchan. This is probably incorrect, as John has a different surname than the other three.
Earlier in Michael's chronicle though, the two offspring not named in the Diane's biography are at least referred to: "their other children are a lawyer and a university professor". This would make the most sense.
Therefore, it would appear that the director of the film (Sarah Polley) has one half-sibling (John) that Diane conceived with her first husband and four half-siblings (Joanna, Mark, a lawyer and a professor) fathered by her second.