It hasn't been as difficult as I'd initially thought it would be, not having our cat here.
I guess once we brought her to my sister's house and saw how she seemed to break away from us to explore and get to know the other cat we realized it actually would be harder on us than it would be on her. It sounds so lame, I know, but it's true. We must have had it in our heads that she would think we were abandonning her, when in reality she didn't seem the least bit worried about us.
The times we've seen her since then she has been calm and cool and has settled right in. I think she is still amazed at how much room there is for her to explore. She still knows us, and is happy to see us, but makes no move to want to come home with us when we leave.