if you think it's mutual then it could be BUT have you considered that her sister might be trying to get one up on your ex?
I have no idea what their relationship is like - is it competitive at all, still?
Just talk to your ex wife, about what you have observed, and how you're thinking of playing it out a bit more. You don't need to ask if it's OK, your gut should tell you that...you do need to make sure everything is communicated properly otherwise you can burn bridges - it's up to the sister whether she wants to burn bridges too.
Talk to both of them in turn...maybe make a move first, then back off, so at least you know for sure that you ain't counting any chickens before they hatch.