What did Kit mean when she said "mercy was the pivot about whom the whole household moved"?

That quote is from chapter 6.  Mercy is a great character.  She is patient, kind, steadfast, loving, caring, etc.  In a lot of ways, Mercy is the exact opposite of Kit.  Kit tries hard, but she definitely is impatient, brash, and, at times, selfish.  It's a little ironic that Mercy is as sweet as she is, because of all of the characters, Mercy has the most "right" to be bitter and angry.  When she was...

That quote is from chapter 6.  Mercy is a great character.  She is patient, kind, steadfast, loving, caring, etc.  In a lot of ways, Mercy is the exact opposite of Kit.  Kit tries hard, but she definitely is impatient, brash, and, at times, selfish.  It's a little ironic that Mercy is as sweet as she is, because of all of the characters, Mercy has the most "right" to be bitter and angry.  When she was younger, she came down with a bad fever.  The fever resulted in crippling one of her legs.  She must use crutches to go anywhere, and isn't able to go all of the places that everybody else can go. Despite that, she is a very hard worker.  Perhaps because of her own suffering, she is able to recognize it in others and offer comfort to people.  The full quote is as follows:

Mercy certainly did not consider herself afflicted. She did a full day’s work and more. Moreover, Kit had soon discovered that Mercy was the pivot about whom the whole household moved. She coaxed her father out of his bitter moods, upheld her timorous and anxious mother, gently restrained her rebellious sister and had reached to draw an uncertain alien into the circle.

A "pivot" is central to any mechanism that turns.  Without a pivot point, a turn is either not made or it isn't exactly smooth.  Describing Mercy as a pivot shows that she is central to the family.  She is an integral part to how it works, especially on an emotional level.  She guides the family with her wisdom and her caring spirit.