There might be two reasons why there was no immune response:

• The immune system responds to protein on cell wall known as Major Histocompatibility Complex. Siblings share many if not all of the important MHC proteins to seem identical to immune system. Maternal twins or identical twins share all of the MHCs. That is one of the reason why there is a high chance of you getting a compatible tissue or organ donation from siblings than parents.
• Sperms and seminal vessels are essentially outside the immune system just like spinal disc, eye lens, joint cartilages. Immune system doesn't even know they are present.

Being incredibly close, genetically, probably helps a lot. Cancer is just ones own cells that turn into a parasitic mass BUT there’s entire dog STI that’s actually a thousands of years old boneless dog(the dog’s cancer cells) which was likely originally spread through a bunch of domesticated, inbred dogs.

So his immune system’s like “eh, close enough”.

Let me preface this by saying I don't know shit about shit, leastways medical shit, so this is probably completely stupid:

His immune system developed like this from the beginning, and it know no other way. So it just works.

Or maybe it doesn't. The person could have all sorts of auto-immune stuff going on, they didn't say.

I wouldn't be surprised if he had autoimmune disorders his whole life, without knowing why.

We've been doing transplants for a while now. Close family are always easier and this happened during development, which possibly makes it even easier for the immune system to recognize as friendly.