One don't be mad at kitty - they don't do it on purose , there is ALWAYS a reason.
If your cat has a litterbox problem you should first consult your vet to rule out any sort of medical reason behind the problem such as a Urinary Track Infection. If your cat is given a clean bill of health and is still having 'accidents' then you can probably be safe in listing it as a behavioral problem.
Now, Cats do not like or handle stress very well. A stressful situation has occurred in your cat's life, your "Sister-In-Law has come to stay". Ideally eliminate the stressor, if possible.
If this is not a possibility some cats, require a short re-training or re-conditioning period. This is accomplished by restricting the cat to a comfortable room with her litter box, food and water (never near the litter box), bed and toys. Confinement should not be considered punishment, and the cat should be visited and played with regularly during this re-training period. Any excursions outside the room should be carefully monitored and gradually lengthened until the cat can be left out with confidence. By offering no opportunity to do the wrong thing, the proper behavior becomes routine.
No housesoiling problem can be completely cured without thoroughly cleaning and neutralizing the soiled areas. Thoroughly clean soiled areas with a strong enzymatic cleaner to remove any lingering odor. You can go the wwww. planeturine.com, they have everything you need.
Hope this helps.