Asalam alikoum sister keep in contact with her as much as you can, try to understand her feelings, even the most beloved of the Prophet salalhu alahi wa salam had problems with jealousy (Aisha radi allah anha) and even Sarah wife of prophet Ibrahim had problems with jealousy against hajar. I dont thin it is any differn than a daughter who sees her father love another woman then her and her mother.
In the furture I am sure she will see that instead of being attracted by christianity she was merely fed up with Islam and more so that maybe she sees that her mother is suffering because of her fathers decision to take another wife. Perhaps it was "the way" in which he married that has her upset. She is young and impressionable and is hurt, so it is understandable she is lashing out like this she doesnt have another outlet for her emotions.
I dont argue that plural marriage is halal, however I think that you must take into consideration all of your families feelings, it can be a huge upset and life changing situation for everyone involved. And if everyone is not on board for the decision I think it more islamic to think of your wife and children whom you find no fault (ie lives lislamically, keeps her duties gives birth) then to just marry hastily "because it is a right" and ALlah know best