Hester is struggling with Pearl during chapter 15. Pearl wants to know what the letter on her mother's breast stands for and why the minister too always puts his hand over his heart. Hester's new sin is not telling her child the whole truth about the letter. Hester even skirts the question by saying that she likes the gold stiching around the letter. Hester avoids Pearl's repeated questions. Hester's sin is lying to her child.
This is worse because in order for children to learn from their parents, they must be able to trust their parents. Pearl will discover the truth... it is like she already has. This lie to her daughter keeps happening as opposed to her singular encounter with Dimmsdale that resulted in little Pearl having life. This lie is giving birth to the evil that is seen in Pearl. Although Hester sought to raise Pearl perfectly, this lie will one day lose Pearl's trust and will make all the other work Hester did in raising her purposeless.