Twin sisters Desiree and Stella Vignes are light-skinned African-Americans in Brit Bennett's novel The Vanishing Half. After the girls run away to New Orleans, their lives take very different paths. Desiree ends up marrying a Black man. Stella, however, decides to try to get a job as a secretary by trying to pass as a white person. The supervisor takes for granted that Stella is white and never questions her about her race. At this point, we get the quotation in question: “She felt queasy at how simple it was. All there was to being white was acting like you were.”
Stella is queasy here because she is nervous. She knows that she isn't telling the whole truth even though she isn't exactly lying. She merely isn't correcting the supervisor's misconception. Then Stella becomes queasy because her deception is working. This opens up a whole new set of opportunities for her. If she can pass as white, she can do things she never would have dreamed. All she has to do is act white; she just has to put on a show. Her light skin will do the rest for her.
Stella's ruse works. In fact, it works so well that Stella ends up marrying her wealthy white boss, Blake Sanders, and entering completely into white society. She never tells him about her true heritage, however, and she is relieved that when their daughter is born, the girl shows no African-American characteristics. Deep down, though, Stella knows that her life is partly a lie, and that is something she will have to face before the novel ends.