Aunt Beryl comes from the Burnell family. This family is of a certain income level, much higher than the Kelveys. Aunt Beryl believes that class distinctions are important. She doesn't think the Burnell children should associate or even talk to the Kelvey children. Aunt Beryl is not alone. The other children at school and even the teachers treat the Kelveys differently. This community, with some exceptions (i. e. Kezia), have embraced the idea that the wealthy families are somehow better than the poorer families. This misguided notion is fully illustrated when Aunt Beryl scolds Lil and Else and drives them away. The point is that Aunt Beryl has this notion that she is better than the Kelveys, simply because the family she comes from has more money. She has a privileged notion of herself and therefore has a warped vision of what being a good person really is. 

It should come as no surprise that she is flawed in other ways. She is terrified by the letter and terrified at the idea that Willie Brent might come to the house: 

A letter had come from Willie Brent, a terrifying, threatening letter, saying if she did not meet him that evening in Pulman's Bush, he'd come to the front door and ask the reason why! But now that she had frightened those little rats of Kelveys and given Kezia a good scolding, her heart felt lighter. That ghastly pressure was gone. She went back to the house humming. 

There is the distinct possibility that Willie is a man she is having some kind of affair with. He wants to meet with her in secret and threatens to come to the house if she does not. She is worried about this and to make herself feel better, she belittles the Kelvey girls. This is a classic case of the narcissistic self-glorifying person who feels better making others feel bad. In asserting her (false) superiority over the Kelveys, she feels better herself.