"How rude of you to interrupt your elders!" Marietta said angrily. What she hates the most is when someone interrupts her, and she actually thought that Aaron wouldn't date to talk in such an indifferent manner as she is one of the elders in the family.
"Interrupt? You are right, I am interrupting you both." Aaron said, however, the tone in his voice didn't change. "However, you are the first one to say that we should finish this meeting as soon as possible, am I right?"
What Aaron is trying to her is that if she wants to finish the meeting early and go back to her original schedule, she should not be trying to make an argument to Martha.
"I wouldn't be talking this much if cousin-in-law Martha didn't start this argument." Knowing what are is getting at, she immediately puts the blame on Martha.