I have had my aunts 3 cats whilst she has been in hospital for the last 6 months (aged 7, 4 & 2). I have 5 of my own and an additional several feral ones which live down on the stable yard. My aunts cats do not get on with my cats at all and have not settled. They have been toileting all over the place including beds, drawers, the bath. We then moved them downstairs (and shut mine up) to keep them away from the rest of the cats but again they were still toileting everywhere. I have even tried putting them outside in a cat pen and shed so they were completely on their own but nothing seems to be helping them settle. I think there are just too many other cats around and the older one especially seems unable to cope. Originally I persevered as it was supposed to be temporary but my aunt is now going to a hospice and obviously can't take the cats back. I've contacted local cat rescues and none have spaces to take them. One suggested I try advertising but who is going to take a cat who is not litter trained? I am now considering PTS. Honestly because I cannot continue to have the house coated in the mess but also because they are quite clearly distressed. They have been checked for UTIS and other illness, we have tried the plug in diffuser, we have had litter trays everywhere - covered and uncovered. Other ideas welcome?
It does seem an extreme solution. They will settle with time.
Did they use trays at your aunts? If not then I would put them in a large dog crate - two if necessary, with a large litter tray in there covering all the space apart from the sleeping area. They will have no choice but to use the trays and so will get used to it.
Please don't have them PTS. They are far too young. Ask several rescue centres to put you on their waiting lists, and stress the urgency of the situation. In a new home, they could be completely different cats. They deserve the chance.