The recent K.M. J. v. K.A. J. Superior Court decision (2015 ONSC 5293) provides a good insight into the factors that parties need to be mindful of when adding third parties to a family law proceeding. Some of the factors that the courts consider are:
a) whether the third party has an interest or claim that will be affected by the outcome of the proceeding;
b) whether the lack of status of a third party in the proceeding will be prejudicial to them or to the parties involved in the proceeding;
c) whether the third party has rights that need to be protected within the context of this proceeding; and,
d) whether the inclusion of third party will unnecessarily complicate the matter or increase the costs for the parties.
In the above noted case, the courts refused to add the parent of the Applicant/father as a third party as they considered it to be unnecessary to do so.
https://www.canlii.org/en/on/onsc/doc/2015/2015onsc5293/2015onsc5293.html