The error is this passage is punctuation. The correct punctuation in these sentences is the following: “Often, when a child talks about fairness, she means sameness. She wants her situation to be the same as her brother's situation.”
A period is needed between the word “sameness” and “she” since it separates two complete sentences. If a period is not included then the whole phrase does not have sense.
Punctuation is an important writing resource that helps us organize and add coherence to our texts.