Cats love cozying up in small spaces. Cats can also be furry little jerks. So, it’s safe to assume that when a cat decides to sleep in the (much larger) bed of a dog sibling, it’s done purely to screw with the dog.
Sadly, dogs are too polite for confrontation and resort to halfhearted pleas with their feline occupier. On the other hand, it appears as if the cats couldn’t care less.
The site calls them jerk cats. I’ve chosen not to use that term, since they’re not necessarily jerks. This would never happen in my home anyway. If Noir is sleeping somewhere–doesn’t matter where–Melchior goes up to him and starts nuzzling, licking, and nibbling. Often he’ll just lie down with his head and neck on Noir. Sometimes they’ll settle into a nice nap. Sometimes Noir gets tired of being harassed and leaves.