It can be a urinary tract infection, do you have new litter, cats are picky about litter, or it could be jealousy, Are there any new additions or changes to the family, or it could be territorial spraying if not fixed, if its that usually fixing won't help if the behavior has already started. You can try the litter that they have that attracts them to the box, or feliway, i didn't have to much luck with feliway but others have. First i would take them to the vet and make sure its not a UTI, that can solve you problem with simple antibiotics. With mine it was extreme jealousy