Here's the novel:
“Your new son-in-law. Do we give him something important, inside the Family?”
He was surprised at the vehemence of the Don’s answer. “Never.” The Don hit the desk with the flat of his hand. “Never. Give him something to earn his living, a good living. But never let him know the Family’s business. Tell the others, Sonny, Fredo, Clemenza.”
Due to his long friendship and co-operation with Vito, Genco probably wouldn't have been as surprised as Tom.
But, pizzaboy, I don't think "Genco would have smelled a rat". At the time of the marriage, Carlo was just a friend of Sonny. Vito was extremely careful.
In a comment on the film, Coppola says that he wanted to have an uglier actress for Connie, because it was easy to fall in love with a beautiful girl. He wanted Carlo to be portrayed as someone who married a not-so-good-looking daughter of a powerful man.
One last thing: Even though it was clear that they wouldn't give him something important, but in fact they really gave him something very unimportant. It must have been humiliating. In a way, the Don created Carlo's hate.
