Help!!My sisters cat keeps peeing on the kitchen floor!!?

My sisters cat keeps peeing on the kitchen floor , the liter box is clean the cats been to the vet and it poos in its box but it wont stop peeing on the floor. the spots have been cleaned...and we put tinfoil down were she peed at to discourage the action so the cat went and peed on her potatoes. Please help we will be getting rid of the cat soon unless we can stop this behavior!