Mother and sister quarrel

It occurs often that my mom and my younger sister quarrel. In this morning, my sister first showed her newly bought clothes to her mom expecting her to say something good. And she asked her "how much is this do you think?" and mom replied "500 yen" which is really cheap, and she was joking I think. And my sister argued back. This triggered their war. My sister didn't like mom's reaction, and my mom didn't like her attitude.
And this war went another direction, they started talking about problems other than clothes. Something like how she behave everyday, money, the fact that mom feeds and give a living to her. It is interesting that they didn't even argue about what was the initial problem.
The first stimulus triggers both of them to express and tell what they had in mind secretly. There is a wall between we think and wanting to express and actually expressing it. And the first stimulus eliminated that wall. In that case, it is like a dam of water, and it won't be easy to stop until they finally finish flowing all of their water(or emotion)
However this is interesting to me, I need to take precaution not to split the relationship of them, and I want them to get along. Now, I think they need the objective viewpoint. And I decided to record their quarrel every time, and let them listen. That will make them realize how their logic collapse, and how ugly they are. With emotion flowing really fast, they are blind, they lose their objective viewpoint.

What am I? I am the observer. Peacemaker.