Ack! Your friends who told you to stick his face in it were wrong. People do this with dogs, but cats are not dogs and do not learn this way. It can make things worse.
Cats do not pee places because they want to make you mad, or make a point...cats don't like to be messy and will use their litterbox if there's nothing else wrong with them, physically or emotionally. Example: my kitty peed on my bed twice, had been great about his litterbox, then I was advised that he had separation anxiety b/c I began locking him out of the bedroom at night and he didn't know why. I fixed the problem by getting him used to a schedule where he played, he got fed, and then we shut the door. He hasn't had a problem since -- kitties love routine, and THIS is something that they understand.
First, trip to the vet to rule out urinary tract problems. That's the number one reason cats go outside their box. Second, wash the 'peed on' with an enzyme cleaner like Get Serious, Nature's miracle, or Petzyme. Kitties can smell their own pee through detergents and soaps, even if you can't. And if they smell it, they'll just pee there again. Like I said, anxiety could be an issue. Has anything changed in your house recently? A move, new people, etc? He might also have anxiety over the punishment. As mentioned, he doesn't understand this.
He might not like his litterbox or the litter, or he might not like sharing with his sister, even if he did before. I'd try getting another kind of litterbox with another kind of litter in it, keep both of them out, and see if this makes a difference. If you have two kitties you should have two litterboxes anyway.
I also tried Feliway -- it worked great. It is facial pheromones in a spray bottle or plug-in diffuser. The diffuser covers a larger area and then you don't need to spray it 2-3 times a day. It helps to calm cats that might have anxiety, and deter the peeing behavior.
