What may have started out as a fondness for your coworker because she reminds you of your sister sounds like it may have changed into romantic feelings. The amount of time you spend thinking about her and wanting to be near her, as well as the difficulties you have concentrating, sound like you are falling in love, or at least infatuation.
I think it’s time to do some soul searching, and possibly talk the situation over with a good friend. Once you have clarified the nature of your feelings, then it may be time to speak to your coworker about it — but not before.
All the best,
Dr. Holly Counts