We're not being harsh if we say that Mrs. Wexler isn't a particularly good mother when it comes to how she treats her younger daughter Turtle. Whereas she constantly lavishes praise on her eldest daughter Angela, who is the perfect child to Grace, she either belittles and criticizes Turtle or ignores her altogether.
Grace isn't just unpleasant towards Turtle, but outright cruel. When inspecting the apartment, Mr. Wexler comes across a bedroom that he suspects might be a closet. Grace immediately proclaims that this will do just fine for Turtle. One might think that a woman who thinks it acceptable for her daughter to have a closet for a bedroom is a mother in name only.
The super-shallow Grace takes against her daughter because, unlike Angela, she's not pretty. Mrs. Wexler is forever criticizing Turtle for making herself look ugly, such as the time when she puts on a scary-looking Halloween costume.
Later on in the story, it seems that Grace is finally starting to treat her youngest daughter with kindness and consideration. She even says that Turtle should wear a particular party dress of hers because she always looks pretty in pink.
But appearances can be deceptive. Grace is only trying to be nice to Turtle so she will share her clues for the Westing Game. Grace hasn't changed at all; she's still the same bad mother to Turtle as she's always been.