When you say "you can tell by her body actions" can you describe what she does? Has she always done it? And how old is she? How many litter trays do you have for the girls, and where do you put them? How often do you clean them, and what kind of litter do you use in them? Have you always used that particular litter?
We got our two girls last November. They were eight weeks old and totally freaked out by people. They're now approaching 11 months old, still skittish, and with more than their fair share of nervousness, but they're making good steady progress.
One of them began soiling an armchair which is on our bedroom. She was peeing on it repeatedly. What she was effectively doing, was marking it as "hers." It was puzzling initially, but because both me and my OH use the chair, and OH used to leave his clothes on it a lot (which consequently also got peed on), it would have smelt strongly of us to her. We believe she was worried that she needed to mark us out as "hers" because of her sister (a bit of a pushy princess) and our other cat, who would also take up our time. At first, we were puzzled what to do, but we hit on a solution which you can try, and see if it works for you.
Given that she's marking on the floor, rather than an item of furniture, I would mark the areas she toilets with some low tack masking tape, so you know exactly where they are.
Clean the areas thoroughly with a specialist enzymatic cleaner. This bit is important. Ordinary cleaners wont work for this type of problem - it has to be an enzymatic cleaner. We used RX66 which we bought online. I think it was a company called "living is easy" but I can't get a link to their company to open.
When you've enzyme cleaned the area, put small dishes of food in those areas, or a bowl of drinking water. Cats will seldom toilet within a close space of where they eat. We kept this up for about two weeks, and it became part of our bedtime routine, to fill up the small bowls - some with dried food, some with wet - only small amounts were necessary, and to take them up to the room with us when we went to bed. It worked for us, and we had no more problems.
The other thing to try, is to think about where the litter trays are located, and how many you have. General rule of thumb is one for each cat, and one spare. They should be placed somewhere the cat can't be cornered when they're looking to enter or exit the tray, or indeed whilst they're using it. Does your other cat intimidate the inappropriate toileter? Does she get interested if the other cat goes to use the tray, and maybe try to block her from entering or leaving the litter tray area?
The other thing is that cats dont always want a "private place" to use the littertray, and it's better to try out new places, even if they wouldnt be your first choice, or it turns out to be a place you'd rather not see or trip over a litter box.
Think about how the cats use the trays - are they perchers, but something is preventing them perching? Are they diggers or scrapers? Is the litter deep enough? Is the litter hurting their paws, or is it a texture they don't like? Is it very dusty, and irritating sensitive eyes or noses? DO you use a tray cleaner which may not agree with the cat who prefers to go outside the box?
What works for one of your cats may not work for them both.
You can experiment using different types of litter in different trays to see which they prefer, especially the more nervous of your two girls.
Is the size and depth of the litter tray an issue? If so, consider a different size or shape. Older or arthritic cats sometimes prefer a tray with lower sides. A young energetic cat might like a deep box where they can scratch and scrape their way to Australia. Don't position a lidded box with the entrance facing into a corner - it might make your cat/s feel uncomfortable about being ambushed.
This might involve a few days of more than usual intimacy with your cat in terms of watching what she does, when she does it, and how she does it, but it really is worth doing if the trick with the food doesnt work.
Let us know how you get on.
