Just a few ideas which might help.
Move the litter tray from the kitchen, especially if you feed the cat in the kitchen. Cats won't toilet where they eat.
Have a litter tray per cat and keep it very clean - remove the poop each time they use it.
Use dilute biological washing powder to clean where the cat wees/poos.
Try a different litter type.
Buy some lemongrass essential oil and put ONE drop on the spot peed or pooped on after thorough cleaning. This is more effective than orange peel.
If you have a cat flap, make sure no strange cats are coming in. Pooing is a very determined territorial marking.
Also, ensure your pregnant sister wears gloves when she handles cat litter.
Hope this helps.