That Sonny was intercepted on his way to his sister's apartment and a trap had been laid at a toll plaza showed that someone must have known the exact timing of his sister's phone call. Pretty much only Carlo could have known this -- it was clear to the audience that Carlo had deliberately started a fight with Connie to get Sonny to drive over.
So I think it was puzzling that after the Sonny's murder and until Michael confronted Carlo, Carlo and Connie not only remained together but had a son. Carlo did not seem concerned at all that Vito or anyone else in the family would figure out something that Carlo knew he had caused.
In a normal marriage, even if Sonny had merely gotten into a legitimate car accident and been simply injured, let alone killed, Connie would have likely been extremely angry with her husband for inadvertently causing Sonny to be injured.
How could Carlo have hoped not to be blamed for leading Sonny into a trap? The only possible explanation for Carlo not being involved was that there was a phone tap, constantly monitored by Barzini's men and when they heard he was coming, they hurriedly arranged the set up at the toll plaza -- this hardly seems likely.
What probably happened is the trap was laid first, hours before Sonny arrived and then Carlo was told somehow to escalate the fight (caused by a female voice on a call to Connie) to the point where Carlo knew Sonny was coming. Or I guess alternatively, Barzini's men would wait all day for Sonny's arrival (seems pretty hard to do this at a toll plaza) and if Sonny did not come, they would try something else -- it actually does not make a lot of sense as a plan to kill him.
But no matter how you look at it, at a minimum the timing of the fight between Connie and Carlo and Sonny being ambushed was extremely suspicious in a situation where suspicion alone might well get one killed.
So, again, why did it appear that life continue as normal for Connie and Carlo rather than Carlo, for example, immediately going into hiding?