@ElfBrony nope. It's not. First of all, what you mean is sexism. Racism implies that what is going on is discriminatory towards people of a different ethnicity and/or includes prejudices based on a person's ethnicity.
Also, it's not sexism. In the context of this Manhwa usually noble women did not cook. It was the job of commoners and very often executed by men. I'm very sure, that in this case, she decided to bake something for her brother because she is trying to manipulate him. She knows that he craves for affection, so what better way is there for her to show how much she "cares" for him, than to do something"extraordinary and unusual"? It's simply smart of her to do that.