Hi Anne, your female cat is most probably doing this as a scent marking behaviour. The reasons could be many & various. for example, a new kitten reaching sexual maturity or adult cat in the neighbourhood area. Or too much pressure on the resources available. Another cat using water & feeding places, resting places, etc. Is the catflap secure? The urine tells all other cats, including her sister, that this is her area & visitors are not welcome! You could explore changing the marking behaviour from urination to scratching by placing a scratching post or device near the doorway & using a pheramone scent spray available from the vets at the wet site. lots of luck.