"Response to catnip is hereditary," says an article in Scientific American,
which rather validates your conclusion, Eliza/Senoufou,
about the immunity of your Siamese pet cats.

More about catnip:
a member of the mint family.
Volatile oil, called nepetalactone, in catnip
is what a cat smells, and it's all about the smell
and what it triggers in
the cat's amygdala and hypothalamus (brain areas).

And Donuel, according to this same article,
the cat responds to catnip as it would
to a feline pheromone --
behaving like a female cat in heat!