K.R.
Poor little thing is stressed out to the max. Since this is a sudden thing that seems to correspond to the sitter, I say you should just do whatever you can to help her through this transition, even if that means letting her sleep in your bed. Obviously it won't interfere with "mom and dad time" since dad is living apart from you all ;)
I had a childhood friend who slept with her mom on and off until she was 14 depending on whether or not mom and dad were together (a whole different stress and story), but my point is that yes, it may lead to her wanting to stay there, but she will learn as she gets older and more comfortable with life changes when it is ok to sleep with mom and when it's not.
You may also ask her and her sister if they want to sleep together?