In general, I would say that Mrs. Wexler treats her daughter Turtle very poorly. Even though parents are supposed to love their kids equally, Mrs. Wexler clearly does not love Turtle as much as she loves her older daughter, Angela (or at least she doesn't treat her like she loves her as much).
Mrs. Wexler always seems to value Angela more than Turtle. She is entranced with Angela because Angela is beautiful. Turtle, on the other hand, is more tomboyish and brainy.
Overall, then, Turtle gets treated like second best because she is not as pretty and feminine as Angela.