A.,
All I can say is from my own experiences, first and foermost DON'T FORCE things, two years is really not that much time. She may still be uncomfortable/unsure with you too. Remember, just like your daughter, her world has drastically changed.
I think divorce with teens is compounded by the normal teen insecurities and makes it harder to develop a relationship, but it can be done. Just be kind, be yourself, be patient and consistent, in the long run that will earn her trust and respect. I know it worked with my boyfriend(husband now) Let her have Dad time too( big mistake on my ex's part). Obviously she needs some healthy socialization with kids her own age,you and her dad should find ways to encourage that.
But you and your boyfriend do need to be on the same page for expectations with both girls, otherwise things can get out of control and needlessly damage relationships.
I told my daughter in ref to her stepmom, you don't have to like her but I do expect you to demonstrate general courtesy and respect. Her dad should expect at least this from her.
It is really good that she gets along with your daughter.
It would be good if you both sat down and asked, both girls separately ,how she would feel if you moved in. Allow her to express herself. She may say things you don't want to hear but perhaps if she sees that you value her opinion anyways, it will do a lot to open communications and trust. I think though by just asking her, she may surprise you. There will need to be some give and take on both sides, but if she knows that she won't be losing her home if you move in that can be helpful too.
Hope some of this helps
A.