Okay this is what I think is happening with my stepdaughter.
All this is symptomatic of you her dad marrying someone and SD failed this time to break this relationship. (Something she has done in the Past).
SD deep down is furious about this. As a rule most women have fled her dad's life as he allowed this from what I can gather so the SD always had a upper hand.
In SD's fury she has taken it upon herself to move away to hurt her dad. This is true because she now knows this does hurt him as he has expressed this.