I'd try isolating her in a bathroom with a box that's lined with newspaper, but no litter in it. See if she'll use that. If she does, add some litter and see how that goes. (Maybe her sister interrupted her once too often while she was pooping.)
Does she strain a lot while pooping? If you think she might be constipated, try adding a few chia seeds to her wet food---maybe 1/4 teaspoon. Soak them first until they get sort of gelatinous.
If you haven't already, do a google search for the problem. I found several articles about it.