Lady Allen is a character mentioned in Sense and Sensibility, and never appears in the book. Her husband's name is unknown but he would be called Sir (First name) Allen.
Lady Allen is the aunt of John Willoughby, as well as his patroness. When she hears of the illegitimate child he had with Eliza Williams, 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 decides to marry the Society heiress, Miss Grey, who has a dowry of £50,000, a fortune.