We adopted a cat that has soiling issues. (We weren’t aware of this when we got him)
He will only use a certain type of litter, has been diagnosed with IBS (after numerous tests to rule out other medical issues) and is on a special diet.
He used to soil outside the box but just in one area so we had puppy pads down. Always did wees in the box. He was worse when we were away so likely linked to anxiety.
It took a few years before we got him using the box totally. Ironically he improved after we got a kitten started to use tray more. Then turning point came when he stayed at a cattery whilst we were away on holiday and he was confined to small space.
He still struggles with going to the loo because of his IBS but his toileting behaviour has massively improved.
We had friends look after our cats in the past but they were fully aware of the issue and we paid them. When his issue got worse and he had to go on medication for a while we found a cattery that were happy to care for him knowing his full care needs along with our other cat.
I would recommend that your sister gets both cats checked out at the vet. Possibly if both are doing it then it is an infection of some sort, if not caused by stress in their environment.
In the meantime gets yourself dispo gloves, puppy pads, wipes and nappy sacks to clear up mess to reduce risk of infection to yourself. Ask your sister for refund of money to cover cost of these.
Tell your sister that it isn’t something she has to resign herself to, it is something that can be fixed with investigation and tweaking what could be potential cause.
These are some of the things to consider…
Cat litter - has she tries different types. Some cats are more sensitive (paws very sensitive) so won’t use if don’t like feel/smell of it.
Cat litter box - does the cat prefers an open or enclosed litter box? Is the box big enough as if too small may not feel comfortable using. Are there more than one box available for cat to use?
Food - has she tries giving different food? Grain free may help if it is digestion/allergy issue.
Medical - need to be seen by vet to rule out infection (parasite) or any other health condition.
Stress - try using plug in calmer. Is the box in a quiet location? Is there tension between cats as soiling outside of box can be a territory tactic.
I hope that the issue with the cats can be resolved, not only for their sakes but for your sister and her family as it’s not pleasant to be living through this situation.
I know a number of posters are saying cat(s) should be rehomed but wanted to share my story so that folk are aware it is something that can be fixed without having to get rid of the cat(s). At the end of the day our pet(s) are our responsibility so it is down to the owner to address the issue and not pass the buck.