“利诱＂？Award her something she likes if she

use big potty? Or, on weekend, give her some watermelon. Then pretend her little potty is broken/lost, maybe she will use the big potty? Or, maybe, she feels the little potty is the only thing she can hold on to as a symbol of being a baby. (In other words, she felt not wanting to grow up as her little sibling gets more attention.) Instead of telling her that she is a bigger sister now, you can tell her that she is still a baby, mom's baby, and spend more alone time with her; maybe even give her some small baby toys, like stuff animal?

In fact, I agree with the other mom -- just wait. She will be ready when she is ready. However, if she tries very hard to avoid using potty in school, it is not good for her.