It might be legally ok... But mere legal compliance is not enough to ensure system safety. TC is hardly a trend setter when it comes to safety... Just look at the FDTL system and the SMS systems which are two giant failures. (SMS being a good idea, but not the Canadian way where they got rid of all the inspectors)
Sunwing are not short of pilots they are correctly crewed for their operation, they need extra pilots for a period of time - it is not sensible from a buisness point of view to employee these pilots year round.
It is a much better idea to contract the services of the required number of pilots from a sister company.
Remember Sunwing is a buisness a buisness is there to make money. Their duty is to provide the best possible return on investment for their shareholders.
You won't win this fight because it has no real basis, that's why each and every time this argument has been raised before it has resulted in..... Nothing, nada, zero zilch!