Final answer:
The question 'How does Kate keep Eva in line?' lacks context for an accurate response, but seems to inquire about a character dynamic possibly related to social stratification or power dynamics within a specific literary work.
Step-by-step explanation:
The question 'How does Kate keep Eva in line?' cannot be accurately answered without additional context. From given reference materials, there are mentions of Kate Middleton and the roles of different individuals in historical or fictional settings but nothing that directly addresses a relationship between a 'Kate' and 'Eva.' It is possible that this question pertains to a specific literary work, a character dynamic, or a representation of social stratification and power dynamics. If this relationship is part of a text being studied, it would be useful to look directly at the material where Kate and Eva interact to understand how one character may influence or control another.