Has she already booked her flights? Did she discuss the possibility of staying for 6 months, with your DH, prior to doing so? If not, then you both need to discuss this with her, and tell her it's simply too much. Or, at least ask how she plans for everything to work out ... what hours will she be working? ... how does she plan to look after her child when she's working? ... make it clear that you have a life of your own and will not be available for childcare whenever she wants/needs it.
If she did discuss it with her father, and he agreed to it, but he did not discuss the facts with you, then you need to have some serious words with him! Tell him, in no uncertain terms, that it's up to him to sort out the logistics of it all with his daughter! It's simply too much to ask, and you can't be expected to give up your life for 6 months just because his daughter wants to get out of the Canadian winter!
You can't be expected to give up your home life, as you know it, for 6 months! I find it shocking that anyone would have the cheek to assume that this is OK! The only time I would agree (reluctantly) to something like this would be if there were some serious issues/marital issues/etc., and she had to move out of the marital home, but it seems she lives with her mother anyway, so that's not the issue here.
Put on your big girl pants, and ask you husband what he intends to do about it, because you're not going to be available for regular childcare, or for all of the other work that's involved when you have people visiting. It's difficult enough when you have visitors staying just for a couple of weeks ... but SIX MONTHS?! ... No, just NO!