There is a topic brought up often about the affect of having not-that-Frum guests over, on your children. I was brought up with many different and interesting guests at our Shabbos table. It never occured to us that their life style had anything to do with us. Basically, it has to do with the “angle of approach”, the mindset in which the newcomer is accepted. When the other children know and fully understand that she must be loved and not imitated, they will hopefully keep things in their proper perspective.
You’d have to work in stages. You start by working only on the relationship. Then, if you have good feelers, you decide as you go along what you can and cannot ask of her. You always want to be one step ahead and still want her to feel dependent on you in as many ways possible.
You might want day to day advice from a Baal Ruach Hakodesh. Don’t forget to Daven, either.