According to Tish in Section Two, why does Ernestine know their father better than she does?

Tish says it is because Ernestine believed since they were little that their father loved Tish more than he loved Ernestine. Ernestine knows now that this is not true, but because of their different personalities, Joseph showed them love in different ways when they were little, and at the time, Ernestine did not understand that.