What does Sal's mom mother mean when she said her cousin can tell her what she is really like? pg 143

Sugar is referring to the way her cousin knows her.... that she will be honest and open about the ways, in which, she has changed.

I could not imagine why she had chosen Idaho. I thought perhaps she had opened an atlas and pointed a finger at any old spot, but later I learned that she had a cousin in Lewiston, Idaho. 'I haven't seen her for fifteen years' my mother said, 'and that's good because she will tell me what I am really like.'