as a previous vet tech 2 things come to mind. 1 These young cats (not kittens anymore) may have urinary tract infections or 2- they are not yet spayed. Either way, they need to go to the vet for an exam. If the family is not cleaning the litter and the poor things are forced to look for alternative spots I feel sad for the cats. However; this is a disgusting behaviour and not acceptable! yuck. I would totally tell them how offensive it is and your concern for your own cats taking on marking behaviour of their own. I'm sorry you have to deal with this. that's gross.