Help! 5-year-old black cat sibling marking all over the house. Possible reasons.

 This is the Reddit.com post supporting the title. What to do?

He has 3000 sq ft of house and attic to roam and play. Has a brother from same litter and both are very socialized. They sleep w us every night in our bed. He has a massive outdoor catio jungle in my backyard (50ft x 80 ft) with a 100ft tunnel that runs under Palms and leads to his lizard hunting paradise. 3 litter boxes scattered throughout the house that are always clean w fresh litter. I’ve tried just about everything… still always cleaning up cat piss from walls, carpets, couch. It’s a nightmare 😱 need advice??!!

My answer would be three possibilities on the basis that the cat has been neutered which is the default situation.

The picture is here to illustrate the page only. This is not the cat in question. Image in the public domain.

Clearly, if the cat is not neutered then there is a much higher chance that they will mark territory. But as mentioned on the presumption that the cat is neutered one possible reason is that the brothers no longer get along. 

Siblings become independent

This may well happen because when siblings in the wild grow up they become independent. They leave the natal home range and go in search of their own home range. Their sibling then becomes just another individual occupying their territory. They are independent cats. They are solitary, essentially. The fact that they are siblings no longer helps to ensure that they get along.

So, this particular individual might feel stressed by the presence of his sibling and is marking territory to send a message to his brother that this is his home range and he is unwanted. Marking territory helps to calm him down because he creates his little home which smells like him.

Although siblings can get along it is not a certainty. The idea that adopting two siblings automatically gives you two cats who will keep each company and get along all their lives is a fallacy.

Separation anxiety?

Another possibility is that the owner is not around enough which is creating separation anxiety in one cat. This is happening despite the fact that the person has created a very nice cat environment with lots of things to do and to stimulate them mentally. That is irrelevant in terms of the particular problem clearly otherwise the problem would not exist.

Medical

A third possibility is that the cat who is said to be spraying is not actually spraying urine but is depositing small spots of bloody urine around the home due to cystitis. This is also heavily linked to stress. Dry cat food may exacerbate the problem.

These are three possibilities. When a cat is spraying inside the home it is invariably to do with a mental issue triggered by stress of some sort. That needs to be addressed. The source of stress needs to be found, isolated and then removed.