Keep in mind, as Wingman already mentioned, if you decide to move your family to St-Jean, it will be done AT YOUR OWN EXPENSE. When you finally do end up getting posted and are due a Cost-Move on behalf of the CF, they will only move your Furniture and Effects (F&E) from your place of enrollment. The only caveat to this would be if St-Jean were closer to your new posting.
Here‘s what a friend of mine did while at St-Jean. When he joined the CF (DEO - pilot), he was married and living in Halifax. He left his wife in their apartment in Halifax while attending BOTC (14 weeks). Once he was posted to the language school - he placed most of his F&E in storage (paid for by the CF) and his wife came out to live with him off-base in a semi-furnished apartment. She attended language school with him, just in a different class. CF spouses are allowed to attend the classes as long as there is ample space. Once he was done with his training, he and his wife were moved from Halifax to Borden and then to Cold Lake.
Make sure you all do your homework BEFORE making a huge decision, it could hurt you in the end.