How many litter trays do you have? In a multi cat household it's recommended that you have one tray per cat plus a spare, placed in different rooms/locations around the house. This is in case one cat claims the litter tray for it's own, if you only have one tray it's possible that one of your cats is banning the cousin from entering. Having said that, I've got 4 cats and have two litter trays. Couldn't stand the thought of 5 litter trays!
Also, what kind of litter are you using. We had problems when we used the paper pellet litter as it was uncomfortable for them to walk on, so they didn't bother.
Finally, your kitten might be attracted to places smelling of his urine and consider them his toilet areas. You can buy sprays from the Vets to neutralise the urine scent or try washing the area in biological washing powder (not bleach based cleaners, they don't work for this).
Apparently, it is possible to train your cat to use the toilet - there was a website showing the steps to take, but it would freak me out seeing my cats use the loo (and I bet they wouldn't flush it or put the seat down after!)