So actually, Mom was right the first time. It was second-guessing herself that got her wrong. Maybe "Actually, Mom, you were right," instead of "no."
(I confess this is a hot button for me; my DH's almost default way to begin an answer is "No, ..." He said something to the driver of the car ahead of us (who couldn't hear anyway, but DH said, "Be careful" or something). I asked, "Oh, did he cut you off?" DH answered, "No, he pulled in too soon.")