I wouldn't be surprised if they did sync up since it's not uncommon for female mammals that live in a group to start cycling at the same time. With rats because they do go in and out of heat so often and get preggers so quickly it's hard to really tell.
I think it would be more likely for two litter sisters to sync rather than unrelated girls. But like Franky said, they go into heat so often it can be hard to tell!