Monsters of paradise

Max and Daniel are twins, and outwardly almost indistinguishable, although their characters are completely different. From childhood, the brothers were separated - at first on the advice of the doctors, and then the parents divorced and "divided" children.
But, after many years, Daniel receives a postcard from Max: the brother undergoes treatment in a private clinic, lost somewhere in the Swiss Alps, and asks him to visit. The place turns out to be truly paradise - picturesque nature, cozy cottages, friendly staff. But just before leaving, the brother turns to Daniel with a strange request.
Why did Daniel agree? Why didn"t I say "no"? After all, then everything would have gone completely differently, and he would not have known that in this "paradise" not angels, but monsters live