We've never seen Remilia actually use her fate manipulation power right? Well, here's why:
When she uses her powers to create a new fate, it overwrites whatever fate the subject previously had. However, she can't see the future, so she has no idea what she's overwriting until she does it. What she can see, however, is all the alternate pasts that she's destroyed. (So, everything between [X instance of using her power] and the present, as it would have been.) The reason she never uses her power anymore is because every night, she's haunted by the image of Sakuya's children.