In The Godfather I, Carlo is seen as an abuser of his wife Connie, and my general feeling (and maybe yours too) is that he's somewhat of an outsider who wants desperately to get involved in the Corleone "family business".
At the end of The Godfather II, we see that Sonny is the one who introduces Carlo to Connie when he brings Carlo to his parents house for his father's birthday party.
For Sonny to have brought Carlo to his parents house for such an occasion, I would think that Sonny trusts Carlo implicitly. You don't just bring a stranger to your father's birthday party, especially when you're in organized crime. Yet there is never, in either movie that I recall, any mention of what Carlo does or how Sonny knows him. In fact, their relationship is extremely poor in The Godfather I.
Does anyone know how Sonny knows Carlo, and what led to their deteriorated relationship?