well thats sad to hear the kitties were kicked out for what....GOING TO THE TOILET IN A HOUSE PLANT gosh thats pretty darn good of them i think rather than on the floor.
If you did not bother to clean out their litter boxes, well thats not very responsible, and the kitties are suffering because of it, maybe you could chat to your mother and ask for a second chance re-taking better care of the cats, and emptying their boxes etc, even draw up a written contract for you and your sister and her to sign, you will know the consquences, as will your unfortunate kitties.
At least if they have to stay outside, then do as the others have suggested get a cat house built for them and assure they have adequate shelter and safety, from the elements and predators.
I hope you will take this advice, because those cats are very much in need of all of the above.