In Chapter 21 of Sense and Sensibility, Jane Austen is using the characters and character traits of Sir John and Lady Middleton to establish the character of Lucy Steele who is a very important character in the novel. In the interactions of Lady Middleton, her children and Lucy Steel, Elinor sees that Lucy Steels has sense, and she perceives readily what kind of sense it is: it is manipulative of circumstances to produce advantages for herself. This is contrasted to Marianne, who wouldn't manipulate anything for any advantage or any cost. The proof is that Elinor continually carries the burden of being civil and courteous in situations that call for tact resulting in her being forced to speak with more "warmth" than she feel (e.g., admiration for Lady Middleton).
Lucy is more sharply contrasted with Elinor who, though genuine and sincere and honest (like Marianne), thinks of the needs of others and can put her own preferences aside--though never her integrity as a later conversation with Lucy demonstrates--in order to give kindness, consideration, civility, or courtesy to another.
Sir John is also shown in contrast to Elinor but is additionally shown in contrast to Marianne and Lucy. Whereas Elinor is accepting, Sir John is unthinkingly accepting. Whereas Marianne is restrained because she will only show her true feelings, Sir John is unrestrained while he shows only his true feeling. Whereas Lucy is congenial and friendly, her foundational traits are based on a shrewd evaluation of what will further her self-interest and are therefore insincere in this regard, but Sir John is congenial and friendly based upon sincere and genuine feelings of expansiveness and generosity that preclude judgment.