Let us remember that we never actally meet Sissy's father in this excellent novel, so we have to analyse their relationship by thinking about what others tell us about them. However, Childers at the circus is able to explain the intense emotional intimacy that was present in their relationship. Note what he says, talking about them:
"Because the two were one. Because they were never asunder. Because, up to this time, he seemed to dote upon her."
He was a man who was hurt more by the fact that his daughter knew of how during every performance he was "goosed" or hissed at than by the actual hissing itself, and loved her so much that he thought his only option was to leave her, now that he had provided for her in terms of giving her a schooling.
However, if we compare this loving relationship, even if it was somewhat misguided, with that of Louisa with her father, we see that the obvious emotional connection that existed between Sissy and her father is completely absent from Mr. Gradgrind's relationship with his daughter. Note the way that Mr. Gradgrind berates Louisa for taking her brother to see the circus:
"You! Thomas and you, to whom the circle of the sciences is open; Thomas and you, who may be said to be replete with facts; Thomas and you, who have been trained to mathematical excactness; Thomas and you, here!" cried Mr. Gradgrind. "In this degraded position! I am amazed."
Mr. Gradgrind only berates Louisa for any sign of anything but facts and reason in her life. When she tries to explain her reason for going, saying that she is "tired" and has been for a long time, she is promptly ignored and silenced, only to be scolded once more a little later. It is clear that openness and honesty is not a feature of this relationship.