I think if you get two kittens about the same age (and not too old) at the same time there's more chance of it as they tend to treat one another as siblings! When I originally got two abyssinians many years ago from different breeders I had the same thing - they bonded like siblings/litter-mates. They'd curl up with my older cat if she'd let them, but she hates them
Yes I know, sometimes the stars are just not aligned for that to happen, which is to say we didn't have the time to pick up two youngsters at the same time. Nonetheless, we are very happy with our cats.