Mrs. Smith is a character mentioned in Sense and Sensibility. She never appears in the book.
She is the aunt of John Willoughby as well as his patroness. Her nephew makes yearly visits to her at her estate of Allenham, which is near Barton and Whitwell.
When she hears of the illegitimate child he had with Eliza Williams, she orders him to marry Eliza, and when he refuses, she disinherits him, leaving him in dire need of an heiress. This is why he cannot marry Marianne Dashwood, even though he is in love with her. He instead marries the heiress Miss Grey, who has a dowry of £50,000, a fortune.
Mrs. Smith eventually reconciles with her nephew and again makes him her heir, which makes him realize too late that he could have had Marianne and a fortune.