There are a number of different ways you can approach this question. You might like to start by thinking of how Cordelia and her sisters are opposed through the play in terms of their character. Expressing this conflict in a sentence might be a good way to finish your essay.
Another thing to think about might be the catastrophic consequences of Lear's inability to perceive the reality of Cordelia's love for him over the appearance of the love that Regan and Goneril protest. This key theme of appearance vs. reality is again something that you could refer to in your final sentence.
Lastly, you might like to consider what Cordelia herself says about the way that her sisters have been unfairly favoured over her true love. In Act I scene 1, she says this as she asks her father to let it be known that she has not been banished because of any dishonour:
If for I want that glib and oily art,
To speak and purpose not...
Cordelia's character is so pure and righteous because of her inability to "speak and purpose not." She is not able to speak untruths, whereas her sisters clearly can. Using Cordelia's own words in your concluding sentence would be a powerful way of summing up the differences between her and her sisters.