Interesting question--and interesting response.

My guess is that Connie was reacting to sudden, violent widowhood--no more husband, no more father to her children. She may have questioned Carlo about the proximity of his beating her, and Sonny's assassination. But, Carlo wouldn't have admitted it to her. And, she'd want to believe her husband. The alternative was to believe not only that her husband was a killer, but was directly involved in a plot to kill her brother--and that her call to Sonny after the beating triggered it.

As an aside: Connie asked Michael to stand godfather to her son. Carlo was whacked on the same day. Connie asked Michael to forgive Fredo. Fredo was whacked after Michael "forgave" him. Not hard to imagine a cause-and-effect train of thought in Connie's mind. Perhap that's why, in III, she said she believed Fredo drowned.